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<div class="section" id="input-and-output">
<span id="tut-io"></span><h1>7. Input and Output<a class="headerlink" href="#input-and-output" title="Permalink to this headline">¶</a></h1>
<p>There are several ways to present the output of a program; data can be printed
in a human-readable form, or written to a file for future use. This chapter will
discuss some of the possibilities.</p>
<div class="section" id="fancier-output-formatting">
<span id="tut-formatting"></span><h2>7.1. Fancier Output Formatting<a class="headerlink" href="#fancier-output-formatting" title="Permalink to this headline">¶</a></h2>
<p>So far we’ve encountered two ways of writing values: <em>expression statements</em> and
the <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal"><span class="pre">print()</span></code></a> function. (A third way is using the <code class="xref py py-meth docutils literal"><span class="pre">write()</span></code> method
of file objects; the standard output file can be referenced as <code class="docutils literal"><span class="pre">sys.stdout</span></code>.
See the Library Reference for more information on this.)</p>
<p>Often you’ll want more control over the formatting of your output than simply
printing space-separated values. There are two ways to format your output; the
first way is to do all the string handling yourself; using string slicing and
concatenation operations you can create any layout you can imagine. The
string type has some methods that perform useful operations for padding
strings to a given column width; these will be discussed shortly. The second
way is to use the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> method.</p>
<p>The <a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal"><span class="pre">string</span></code></a> module contains a <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">Template</span></code></a> class which offers
yet another way to substitute values into strings.</p>
<p>One question remains, of course: how do you convert values to strings? Luckily,
Python has ways to convert any value to a string: pass it to the <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal"><span class="pre">repr()</span></code></a>
or <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal"><span class="pre">str()</span></code></a> functions.</p>
<p>The <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal"><span class="pre">str()</span></code></a> function is meant to return representations of values which are
fairly human-readable, while <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal"><span class="pre">repr()</span></code></a> is meant to generate representations
which can be read by the interpreter (or will force a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a> if
there is no equivalent syntax). For objects which don’t have a particular
representation for human consumption, <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal"><span class="pre">str()</span></code></a> will return the same value as
<a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal"><span class="pre">repr()</span></code></a>. Many values, such as numbers or structures like lists and
dictionaries, have the same representation using either function. Strings, in
particular, have two distinct representations.</p>
<p>Some examples:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'Hello, world.'</span>
<span class="gp">>>> </span><span class="nb">str</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="go">'Hello, world.'</span>
<span class="gp">>>> </span><span class="nb">repr</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="go">"'Hello, world.'"</span>
<span class="gp">>>> </span><span class="nb">str</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="mi">7</span><span class="p">)</span>
<span class="go">'0.14285714285714285'</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span> <span class="o">*</span> <span class="mf">3.25</span>
<span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="mi">200</span> <span class="o">*</span> <span class="mi">200</span>
<span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'The value of x is '</span> <span class="o">+</span> <span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">+</span> <span class="s1">', and y is '</span> <span class="o">+</span> <span class="nb">repr</span><span class="p">(</span><span class="n">y</span><span class="p">)</span> <span class="o">+</span> <span class="s1">'...'</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="go">The value of x is 32.5, and y is 40000...</span>
<span class="gp">>>> </span><span class="c1"># The repr() of a string adds string quotes and backslashes:</span>
<span class="gp">... </span><span class="n">hello</span> <span class="o">=</span> <span class="s1">'hello, world</span><span class="se">\n</span><span class="s1">'</span>
<span class="gp">>>> </span><span class="n">hellos</span> <span class="o">=</span> <span class="nb">repr</span><span class="p">(</span><span class="n">hello</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">hellos</span><span class="p">)</span>
<span class="go">'hello, world\n'</span>
<span class="gp">>>> </span><span class="c1"># The argument to repr() may be any Python object:</span>
<span class="gp">... </span><span class="nb">repr</span><span class="p">((</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">)))</span>
<span class="go">"(32.5, 40000, ('spam', 'eggs'))"</span>
</pre></div>
</div>
<p>Here are two ways to write a table of squares and cubes:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">11</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="p">)</span><span class="o">.</span><span class="n">rjust</span><span class="p">(</span><span class="mi">2</span><span class="p">),</span> <span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="p">)</span><span class="o">.</span><span class="n">rjust</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp">... </span> <span class="c1"># Note use of 'end' on previous line</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="p">)</span><span class="o">.</span><span class="n">rjust</span><span class="p">(</span><span class="mi">4</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go"> 1 1 1</span>
<span class="go"> 2 4 8</span>
<span class="go"> 3 9 27</span>
<span class="go"> 4 16 64</span>
<span class="go"> 5 25 125</span>
<span class="go"> 6 36 216</span>
<span class="go"> 7 49 343</span>
<span class="go"> 8 64 512</span>
<span class="go"> 9 81 729</span>
<span class="go">10 100 1000</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">11</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{0:2d}</span><span class="s1"> </span><span class="si">{1:3d}</span><span class="s1"> </span><span class="si">{2:4d}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go"> 1 1 1</span>
<span class="go"> 2 4 8</span>
<span class="go"> 3 9 27</span>
<span class="go"> 4 16 64</span>
<span class="go"> 5 25 125</span>
<span class="go"> 6 36 216</span>
<span class="go"> 7 49 343</span>
<span class="go"> 8 64 512</span>
<span class="go"> 9 81 729</span>
<span class="go">10 100 1000</span>
</pre></div>
</div>
<p>(Note that in the first example, one space between each column was added by the
way <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal"><span class="pre">print()</span></code></a> works: it always adds spaces between its arguments.)</p>
<p>This example demonstrates the <a class="reference internal" href="../library/stdtypes.html#str.rjust" title="str.rjust"><code class="xref py py-meth docutils literal"><span class="pre">str.rjust()</span></code></a> method of string
objects, which right-justifies a string in a field of a given width by padding
it with spaces on the left. There are similar methods <a class="reference internal" href="../library/stdtypes.html#str.ljust" title="str.ljust"><code class="xref py py-meth docutils literal"><span class="pre">str.ljust()</span></code></a> and
<a class="reference internal" href="../library/stdtypes.html#str.center" title="str.center"><code class="xref py py-meth docutils literal"><span class="pre">str.center()</span></code></a>. These methods do not write anything, they just return a
new string. If the input string is too long, they don’t truncate it, but
return it unchanged; this will mess up your column lay-out but that’s usually
better than the alternative, which would be lying about a value. (If you
really want truncation you can always add a slice operation, as in
<code class="docutils literal"><span class="pre">x.ljust(n)[:n]</span></code>.)</p>
<p>There is another method, <a class="reference internal" href="../library/stdtypes.html#str.zfill" title="str.zfill"><code class="xref py py-meth docutils literal"><span class="pre">str.zfill()</span></code></a>, which pads a numeric string on the
left with zeros. It understands about plus and minus signs:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'12'</span><span class="o">.</span><span class="n">zfill</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="go">'00012'</span>
<span class="gp">>>> </span><span class="s1">'-3.14'</span><span class="o">.</span><span class="n">zfill</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span>
<span class="go">'-003.14'</span>
<span class="gp">>>> </span><span class="s1">'3.14159265359'</span><span class="o">.</span><span class="n">zfill</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="go">'3.14159265359'</span>
</pre></div>
</div>
<p>Basic usage of the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> method looks like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'We are the </span><span class="si">{}</span><span class="s1"> who say "</span><span class="si">{}</span><span class="s1">!"'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="s1">'knights'</span><span class="p">,</span> <span class="s1">'Ni'</span><span class="p">))</span>
<span class="go">We are the knights who say "Ni!"</span>
</pre></div>
</div>
<p>The brackets and characters within them (called format fields) are replaced with
the objects passed into the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> method. A number in the
brackets can be used to refer to the position of the object passed into the
<a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> method.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{0}</span><span class="s1"> and </span><span class="si">{1}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">))</span>
<span class="go">spam and eggs</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{1}</span><span class="s1"> and </span><span class="si">{0}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">))</span>
<span class="go">eggs and spam</span>
</pre></div>
</div>
<p>If keyword arguments are used in the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> method, their values
are referred to by using the name of the argument.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'This </span><span class="si">{food}</span><span class="s1"> is </span><span class="si">{adjective}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span>
<span class="gp">... </span> <span class="n">food</span><span class="o">=</span><span class="s1">'spam'</span><span class="p">,</span> <span class="n">adjective</span><span class="o">=</span><span class="s1">'absolutely horrible'</span><span class="p">))</span>
<span class="go">This spam is absolutely horrible.</span>
</pre></div>
</div>
<p>Positional and keyword arguments can be arbitrarily combined:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'The story of </span><span class="si">{0}</span><span class="s1">, </span><span class="si">{1}</span><span class="s1">, and </span><span class="si">{other}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="s1">'Bill'</span><span class="p">,</span> <span class="s1">'Manfred'</span><span class="p">,</span>
<span class="go"> other='Georg'))</span>
<span class="go">The story of Bill, Manfred, and Georg.</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">'!a'</span></code> (apply <a class="reference internal" href="../library/functions.html#ascii" title="ascii"><code class="xref py py-func docutils literal"><span class="pre">ascii()</span></code></a>), <code class="docutils literal"><span class="pre">'!s'</span></code> (apply <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal"><span class="pre">str()</span></code></a>) and <code class="docutils literal"><span class="pre">'!r'</span></code>
(apply <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal"><span class="pre">repr()</span></code></a>) can be used to convert the value before it is formatted:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">contents</span> <span class="o">=</span> <span class="s1">'eels'</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'My hovercraft is full of </span><span class="si">{}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">contents</span><span class="p">))</span>
<span class="go">My hovercraft is full of eels.</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'My hovercraft is full of </span><span class="si">{!r}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">contents</span><span class="p">))</span>
<span class="go">My hovercraft is full of 'eels'.</span>
</pre></div>
</div>
<p>An optional <code class="docutils literal"><span class="pre">':'</span></code> and format specifier can follow the field name. This allows
greater control over how the value is formatted. The following example
rounds Pi to three places after the decimal.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">math</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'The value of PI is approximately </span><span class="si">{0:.3f}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">math</span><span class="o">.</span><span class="n">pi</span><span class="p">))</span>
<span class="go">The value of PI is approximately 3.142.</span>
</pre></div>
</div>
<p>Passing an integer after the <code class="docutils literal"><span class="pre">':'</span></code> will cause that field to be a minimum
number of characters wide. This is useful for making tables pretty.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">table</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'Sjoerd'</span><span class="p">:</span> <span class="mi">4127</span><span class="p">,</span> <span class="s1">'Jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s1">'Dcab'</span><span class="p">:</span> <span class="mi">7678</span><span class="p">}</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">name</span><span class="p">,</span> <span class="n">phone</span> <span class="ow">in</span> <span class="n">table</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{0:10}</span><span class="s1"> ==> </span><span class="si">{1:10d}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">phone</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go">Jack ==> 4098</span>
<span class="go">Dcab ==> 7678</span>
<span class="go">Sjoerd ==> 4127</span>
</pre></div>
</div>
<p>If you have a really long format string that you don’t want to split up, it
would be nice if you could reference the variables to be formatted by name
instead of by position. This can be done by simply passing the dict and using
square brackets <code class="docutils literal"><span class="pre">'[]'</span></code> to access the keys</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">table</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'Sjoerd'</span><span class="p">:</span> <span class="mi">4127</span><span class="p">,</span> <span class="s1">'Jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s1">'Dcab'</span><span class="p">:</span> <span class="mi">8637678</span><span class="p">}</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'Jack: </span><span class="si">{0[Jack]:d}</span><span class="s1">; Sjoerd: </span><span class="si">{0[Sjoerd]:d}</span><span class="s1">; '</span>
<span class="gp">... </span> <span class="s1">'Dcab: </span><span class="si">{0[Dcab]:d}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">table</span><span class="p">))</span>
<span class="go">Jack: 4098; Sjoerd: 4127; Dcab: 8637678</span>
</pre></div>
</div>
<p>This could also be done by passing the table as keyword arguments with the ‘**’
notation.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">table</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'Sjoerd'</span><span class="p">:</span> <span class="mi">4127</span><span class="p">,</span> <span class="s1">'Jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s1">'Dcab'</span><span class="p">:</span> <span class="mi">8637678</span><span class="p">}</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'Jack: </span><span class="si">{Jack:d}</span><span class="s1">; Sjoerd: </span><span class="si">{Sjoerd:d}</span><span class="s1">; Dcab: </span><span class="si">{Dcab:d}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="o">**</span><span class="n">table</span><span class="p">))</span>
<span class="go">Jack: 4098; Sjoerd: 4127; Dcab: 8637678</span>
</pre></div>
</div>
<p>This is particularly useful in combination with the built-in function
<a class="reference internal" href="../library/functions.html#vars" title="vars"><code class="xref py py-func docutils literal"><span class="pre">vars()</span></code></a>, which returns a dictionary containing all local variables.</p>
<p>For a complete overview of string formatting with <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a>, see
<a class="reference internal" href="../library/string.html#formatstrings"><span>Format String Syntax</span></a>.</p>
<div class="section" id="old-string-formatting">
<h3>7.1.1. Old string formatting<a class="headerlink" href="#old-string-formatting" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal"><span class="pre">%</span></code> operator can also be used for string formatting. It interprets the
left argument much like a <code class="xref c c-func docutils literal"><span class="pre">sprintf()</span></code>-style format string to be applied
to the right argument, and returns the string resulting from this formatting
operation. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">math</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'The value of PI is approximately </span><span class="si">%5.3f</span><span class="s1">.'</span> <span class="o">%</span> <span class="n">math</span><span class="o">.</span><span class="n">pi</span><span class="p">)</span>
<span class="go">The value of PI is approximately 3.142.</span>
</pre></div>
</div>
<p>More information can be found in the <a class="reference internal" href="../library/stdtypes.html#old-string-formatting"><span>printf-style String Formatting</span></a> section.</p>
</div>
</div>
<div class="section" id="reading-and-writing-files">
<span id="tut-files"></span><h2>7.2. Reading and Writing Files<a class="headerlink" href="#reading-and-writing-files" title="Permalink to this headline">¶</a></h2>
<p id="index-0"><a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a> returns a <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a>, and is most commonly used with
two arguments: <code class="docutils literal"><span class="pre">open(filename,</span> <span class="pre">mode)</span></code>.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'workfile'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span>
</pre></div>
</div>
<p>The first argument is a string containing the filename. The second argument is
another string containing a few characters describing the way in which the file
will be used. <em>mode</em> can be <code class="docutils literal"><span class="pre">'r'</span></code> when the file will only be read, <code class="docutils literal"><span class="pre">'w'</span></code>
for only writing (an existing file with the same name will be erased), and
<code class="docutils literal"><span class="pre">'a'</span></code> opens the file for appending; any data written to the file is
automatically added to the end. <code class="docutils literal"><span class="pre">'r+'</span></code> opens the file for both reading and
writing. The <em>mode</em> argument is optional; <code class="docutils literal"><span class="pre">'r'</span></code> will be assumed if it’s
omitted.</p>
<p>Normally, files are opened in <em class="dfn">text mode</em>, that means, you read and write
strings from and to the file, which are encoded in a specific encoding. If
encoding is not specified, the default is platform dependent (see
<a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a>). <code class="docutils literal"><span class="pre">'b'</span></code> appended to the mode opens the file in
<em class="dfn">binary mode</em>: now the data is read and written in the form of bytes
objects. This mode should be used for all files that don’t contain text.</p>
<p>In text mode, the default when reading is to convert platform-specific line
endings (<code class="docutils literal"><span class="pre">\n</span></code> on Unix, <code class="docutils literal"><span class="pre">\r\n</span></code> on Windows) to just <code class="docutils literal"><span class="pre">\n</span></code>. When writing in
text mode, the default is to convert occurrences of <code class="docutils literal"><span class="pre">\n</span></code> back to
platform-specific line endings. This behind-the-scenes modification
to file data is fine for text files, but will corrupt binary data like that in
<code class="file docutils literal"><span class="pre">JPEG</span></code> or <code class="file docutils literal"><span class="pre">EXE</span></code> files. Be very careful to use binary mode when
reading and writing such files.</p>
<div class="section" id="methods-of-file-objects">
<span id="tut-filemethods"></span><h3>7.2.1. Methods of File Objects<a class="headerlink" href="#methods-of-file-objects" title="Permalink to this headline">¶</a></h3>
<p>The rest of the examples in this section will assume that a file object called
<code class="docutils literal"><span class="pre">f</span></code> has already been created.</p>
<p>To read a file’s contents, call <code class="docutils literal"><span class="pre">f.read(size)</span></code>, which reads some quantity of
data and returns it as a string (in text mode) or bytes object (in binary mode).
<em>size</em> is an optional numeric argument. When <em>size</em> is omitted or negative, the
entire contents of the file will be read and returned; it’s your problem if the
file is twice as large as your machine’s memory. Otherwise, at most <em>size</em> bytes
are read and returned.
If the end of the file has been reached, <code class="docutils literal"><span class="pre">f.read()</span></code> will return an empty
string (<code class="docutils literal"><span class="pre">''</span></code>).</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="go">'This is the entire file.\n'</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="go">''</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">f.readline()</span></code> reads a single line from the file; a newline character (<code class="docutils literal"><span class="pre">\n</span></code>)
is left at the end of the string, and is only omitted on the last line of the
file if the file doesn’t end in a newline. This makes the return value
unambiguous; if <code class="docutils literal"><span class="pre">f.readline()</span></code> returns an empty string, the end of the file
has been reached, while a blank line is represented by <code class="docutils literal"><span class="pre">'\n'</span></code>, a string
containing only a single newline.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">'This is the first line of the file.\n'</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">'Second line of the file\n'</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">''</span>
</pre></div>
</div>
<p>For reading lines from a file, you can loop over the file object. This is memory
efficient, fast, and leads to simple code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">''</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">This is the first line of the file.</span>
<span class="go">Second line of the file</span>
</pre></div>
</div>
<p>If you want to read all the lines of a file in a list you can also use
<code class="docutils literal"><span class="pre">list(f)</span></code> or <code class="docutils literal"><span class="pre">f.readlines()</span></code>.</p>
<p><code class="docutils literal"><span class="pre">f.write(string)</span></code> writes the contents of <em>string</em> to the file, returning
the number of characters written.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'This is a test</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="go">15</span>
</pre></div>
</div>
<p>Other types of objects need to be converted – either to a string (in text mode)
or a bytes object (in binary mode) – before writing them:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">value</span> <span class="o">=</span> <span class="p">(</span><span class="s1">'the answer'</span><span class="p">,</span> <span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="c1"># convert the tuple to string</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="go">18</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">f.tell()</span></code> returns an integer giving the file object’s current position in the file
represented as number of bytes from the beginning of the file when in binary mode and
an opaque number when in text mode.</p>
<p>To change the file object’s position, use <code class="docutils literal"><span class="pre">f.seek(offset,</span> <span class="pre">from_what)</span></code>. The position is computed
from adding <em>offset</em> to a reference point; the reference point is selected by
the <em>from_what</em> argument. A <em>from_what</em> value of 0 measures from the beginning
of the file, 1 uses the current file position, and 2 uses the end of the file as
the reference point. <em>from_what</em> can be omitted and defaults to 0, using the
beginning of the file as the reference point.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'workfile'</span><span class="p">,</span> <span class="s1">'rb+'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">b</span><span class="s1">'0123456789abcdef'</span><span class="p">)</span>
<span class="go">16</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># Go to the 6th byte in the file</span>
<span class="go">5</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="go">b'5'</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="c1"># Go to the 3rd byte before the end</span>
<span class="go">13</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="go">b'd'</span>
</pre></div>
</div>
<p>In text files (those opened without a <code class="docutils literal"><span class="pre">b</span></code> in the mode string), only seeks
relative to the beginning of the file are allowed (the exception being seeking
to the very file end with <code class="docutils literal"><span class="pre">seek(0,</span> <span class="pre">2)</span></code>) and the only valid <em>offset</em> values are
those returned from the <code class="docutils literal"><span class="pre">f.tell()</span></code>, or zero. Any other <em>offset</em> value produces
undefined behaviour.</p>
<p>When you’re done with a file, call <code class="docutils literal"><span class="pre">f.close()</span></code> to close it and free up any
system resources taken up by the open file. After calling <code class="docutils literal"><span class="pre">f.close()</span></code>,
attempts to use the file object will automatically fail.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n">?</span>
<span class="gr">ValueError</span>: <span class="n">I/O operation on closed file</span>
</pre></div>
</div>
<p>It is good practice to use the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal"><span class="pre">with</span></code></a> keyword when dealing with file
objects. This has the advantage that the file is properly closed after its
suite finishes, even if an exception is raised on the way. It is also much
shorter than writing equivalent <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></a>-<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal"><span class="pre">finally</span></code></a> blocks:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'workfile'</span><span class="p">,</span> <span class="s1">'r'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">read_data</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="n">closed</span>
<span class="go">True</span>
</pre></div>
</div>
<p>File objects have some additional methods, such as <code class="xref py py-meth docutils literal"><span class="pre">isatty()</span></code> and
<code class="xref py py-meth docutils literal"><span class="pre">truncate()</span></code> which are less frequently used; consult the Library
Reference for a complete guide to file objects.</p>
</div>
<div class="section" id="saving-structured-data-with-json">
<span id="tut-json"></span><h3>7.2.2. Saving structured data with <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal"><span class="pre">json</span></code></a><a class="headerlink" href="#saving-structured-data-with-json" title="Permalink to this headline">¶</a></h3>
<p id="index-1">Strings can easily be written to and read from a file. Numbers take a bit more
effort, since the <code class="xref py py-meth docutils literal"><span class="pre">read()</span></code> method only returns strings, which will have to
be passed to a function like <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal"><span class="pre">int()</span></code></a>, which takes a string like <code class="docutils literal"><span class="pre">'123'</span></code>
and returns its numeric value 123. When you want to save more complex data
types like nested lists and dictionaries, parsing and serializing by hand
becomes complicated.</p>
<p>Rather than having users constantly writing and debugging code to save
complicated data types to files, Python allows you to use the popular data
interchange format called <a class="reference external" href="http://json.org">JSON (JavaScript Object Notation)</a>. The standard module called <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal"><span class="pre">json</span></code></a> can take Python
data hierarchies, and convert them to string representations; this process is
called <em class="dfn">serializing</em>. Reconstructing the data from the string representation
is called <em class="dfn">deserializing</em>. Between serializing and deserializing, the
string representing the object may have been stored in a file or data, or
sent over a network connection to some distant machine.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The JSON format is commonly used by modern applications to allow for data
exchange. Many programmers are already familiar with it, which makes
it a good choice for interoperability.</p>
</div>
<p>If you have an object <code class="docutils literal"><span class="pre">x</span></code>, you can view its JSON string representation with a
simple line of code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'simple'</span><span class="p">,</span> <span class="s1">'list'</span><span class="p">])</span>
<span class="go">'[1, "simple", "list"]'</span>
</pre></div>
</div>
<p>Another variant of the <a class="reference internal" href="../library/json.html#json.dumps" title="json.dumps"><code class="xref py py-func docutils literal"><span class="pre">dumps()</span></code></a> function, called <a class="reference internal" href="../library/json.html#json.dump" title="json.dump"><code class="xref py py-func docutils literal"><span class="pre">dump()</span></code></a>,
simply serializes the object to a <a class="reference internal" href="../glossary.html#term-text-file"><span class="xref std std-term">text file</span></a>. So if <code class="docutils literal"><span class="pre">f</span></code> is a
<a class="reference internal" href="../glossary.html#term-text-file"><span class="xref std std-term">text file</span></a> object opened for writing, we can do this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">json</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">f</span><span class="p">)</span>
</pre></div>
</div>
<p>To decode the object again, if <code class="docutils literal"><span class="pre">f</span></code> is a <a class="reference internal" href="../glossary.html#term-text-file"><span class="xref std std-term">text file</span></a> object which has
been opened for reading:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>
</div>
<p>This simple serialization technique can handle lists and dictionaries, but
serializing arbitrary class instances in JSON requires a bit of extra effort.
The reference for the <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal"><span class="pre">json</span></code></a> module contains an explanation of this.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal"><span class="pre">pickle</span></code></a> - the pickle module</p>
<p class="last">Contrary to <a class="reference internal" href="#tut-json"><span>JSON</span></a>, <em>pickle</em> is a protocol which allows
the serialization of arbitrarily complex Python objects. As such, it is
specific to Python and cannot be used to communicate with applications
written in other languages. It is also insecure by default:
deserializing pickle data coming from an untrusted source can execute
arbitrary code, if the data was crafted by a skilled attacker.</p>
</div>
</div>
</div>
</div>
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<li><a class="reference internal" href="#">7. Input and Output</a><ul>
<li><a class="reference internal" href="#fancier-output-formatting">7.1. Fancier Output Formatting</a><ul>
<li><a class="reference internal" href="#old-string-formatting">7.1.1. Old string formatting</a></li>
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<li><a class="reference internal" href="#reading-and-writing-files">7.2. Reading and Writing Files</a><ul>
<li><a class="reference internal" href="#methods-of-file-objects">7.2.1. Methods of File Objects</a></li>
<li><a class="reference internal" href="#saving-structured-data-with-json">7.2.2. Saving structured data with <code class="docutils literal"><span class="pre">json</span></code></a></li>
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