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<li>Decomposing and composing units</li>
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<div class="section" id="decomposing-and-composing-units">
<h1>Decomposing and composing units<a class="headerlink" href="#decomposing-and-composing-units" title="Permalink to this headline">¶</a></h1>
<div class="section" id="reducing-a-unit-to-its-irreducible-parts">
<h2>Reducing a unit to its irreducible parts<a class="headerlink" href="#reducing-a-unit-to-its-irreducible-parts" title="Permalink to this headline">¶</a></h2>
<p>A unit can be decomposed into its irreducible parts using the
<a class="reference internal" href="../_generated/astropy.units.core.UnitBase.html#astropy.units.core.UnitBase.decompose" title="astropy.units.core.UnitBase.decompose"><tt class="xref py py-obj docutils literal"><span class="pre">decompose</span></tt></a> method:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">astropy</span> <span class="kn">import</span> <span class="n">units</span> <span class="k">as</span> <span class="n">u</span>
<span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Ry</span>
<span class="go">Unit("Ry")</span>
<span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Ry</span><span class="o">.</span><span class="n">decompose</span><span class="p">()</span>
<span class="go">Unit("2.18e-18 m2 kg / (s2)")</span>
</pre></div>
</div>
<p>You can limit the selection of units that you want to decompose to
using the <tt class="xref py py-obj docutils literal"><span class="pre">bases</span></tt> keyword argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Ry</span><span class="o">.</span><span class="n">decompose</span><span class="p">(</span><span class="n">bases</span><span class="o">=</span><span class="p">[</span><span class="n">u</span><span class="o">.</span><span class="n">m</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">N</span><span class="p">])</span>
<span class="go">Unit("2.179872e-18 m N")</span>
</pre></div>
</div>
<p>This is also useful to decompose to a particular system. For example,
to decompose the Rydberg unit in terms of CGS units:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Ry</span><span class="o">.</span><span class="n">decompose</span><span class="p">(</span><span class="n">bases</span><span class="o">=</span><span class="n">u</span><span class="o">.</span><span class="n">cgs</span><span class="o">.</span><span class="n">bases</span><span class="p">)</span>
<span class="go">Unit("2.179872e-11 cm2 g / (s2)")</span>
</pre></div>
</div>
</div>
<div class="section" id="automatically-composing-a-unit-into-more-complex-units">
<h2>Automatically composing a unit into more complex units<a class="headerlink" href="#automatically-composing-a-unit-into-more-complex-units" title="Permalink to this headline">¶</a></h2>
<p>Conversely, a unit may be recomposed back into more complex units
using the <a class="reference internal" href="../_generated/astropy.units.core.UnitBase.html#astropy.units.core.UnitBase.compose" title="astropy.units.core.UnitBase.compose"><tt class="xref py py-obj docutils literal"><span class="pre">compose</span></tt></a> method. Since there
may be multiple equally good results, a list is always returned:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">u</span><span class="o">.</span><span class="n">Ry</span><span class="o">.</span><span class="n">decompose</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">x</span><span class="o">.</span><span class="n">compose</span><span class="p">()</span>
<span class="go">[Unit("1.000000e+00 Ry"),</span>
<span class="go"> Unit("5.210019e-22 kcal"),</span>
<span class="go"> Unit("5.210019e-19 cal"),</span>
<span class="go"> Unit("2.066120e-21 BTU"),</span>
<span class="go"> Unit("1.360569e+01 eV"),</span>
<span class="go"> Unit("2.179872e-18 J"),</span>
<span class="go"> Unit("2.179872e-11 erg")]</span>
</pre></div>
</div>
<p>Some other interesting examples:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="p">(</span><span class="n">u</span><span class="o">.</span><span class="n">s</span> <span class="o">**</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span><span class="o">.</span><span class="n">compose</span><span class="p">()</span>
<span class="go">[Unit("Hz"),</span>
<span class="go"> Unit("1 / (s)"),</span>
<span class="go"> Unit("3.155693e+07 / (yr)"),</span>
<span class="go"> Unit("8.640000e+04 / (d)"),</span>
<span class="go"> Unit("6.000000e+01 / (min)"),</span>
<span class="go"> Unit("3.600000e+03 / (h)"),</span>
<span class="go"> Unit("6.048000e+05 / (wk)"),</span>
<span class="go"> Unit("3.155693e+07 / (a)"),</span>
<span class="go"> Unit("8.616409e+04 / (sday)"),</span>
<span class="go"> Unit("1.209600e+06 / (fortnight)")]</span>
</pre></div>
</div>
<p>Composition can be combined with <a class="reference internal" href="equivalencies.html#unit-equivalencies"><em>Equivalencies</em></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="p">(</span><span class="n">u</span><span class="o">.</span><span class="n">s</span> <span class="o">**</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span><span class="o">.</span><span class="n">compose</span><span class="p">(</span><span class="n">equivalencies</span><span class="o">=</span><span class="n">u</span><span class="o">.</span><span class="n">spectral</span><span class="p">())</span>
<span class="go">[Unit("Hz"),</span>
<span class="go"> Unit("J"),</span>
<span class="go"> Unit("m"),</span>
<span class="go"> Unit("1 / (s)"),</span>
<span class="go"> Unit("2.390057e-01 cal"),</span>
<span class="go"> Unit("1.057001e-16 lyr"),</span>
<span class="go"> Unit("1.093613e+00 yd"),</span>
<span class="go"> Unit("3.240779e-17 pc"),</span>
<span class="go"> Unit("4.587425e+17 Ry"),</span>
<span class="go"> Unit("1.000000e+06 micron"),</span>
<span class="go"> Unit("3.937008e+01 inch"),</span>
<span class="go"> Unit("6.684587e-12 AU"),</span>
<span class="go"> Unit("1.000000e+02 cm"),</span>
<span class="go"> Unit("1.437798e-09 solRad"),</span>
<span class="go"> Unit("6.241509e+18 eV"),</span>
<span class="go"> Unit("3.280840e+00 ft"),</span>
<span class="go"> Unit("2.390057e-04 kcal"),</span>
<span class="go"> Unit("1.000000e+10 Angstrom"),</span>
<span class="go"> Unit("1.000000e+07 erg"),</span>
<span class="go"> Unit("9.478171e-04 BTU"),</span>
<span class="go"> Unit("6.213712e-04 mi"),</span>
<span class="go"> Unit("6.000000e+01 / (min)"),</span>
<span class="go"> Unit("3.600000e+03 / (h)"),</span>
<span class="go"> Unit("6.048000e+05 / (wk)"),</span>
<span class="go"> Unit("8.616409e+04 / (sday)"),</span>
<span class="go"> Unit("1.209600e+06 / (fortnight)"),</span>
<span class="go"> Unit("8.640000e+04 / (d)"),</span>
<span class="go"> Unit("3.155693e+07 / (yr)"),</span>
<span class="go"> Unit("3.155693e+07 / (a)")]</span>
</pre></div>
</div>
<p>Obviously a name doesn’t exist for every arbitrary derived unit
imaginable. In that case, the system will do its best to reduce the
unit to the fewest possible symbols:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="p">(</span><span class="n">u</span><span class="o">.</span><span class="n">cd</span> <span class="o">*</span> <span class="n">u</span><span class="o">.</span><span class="n">sr</span> <span class="o">*</span> <span class="n">u</span><span class="o">.</span><span class="n">V</span> <span class="o">*</span> <span class="n">u</span><span class="o">.</span><span class="n">s</span><span class="p">)</span><span class="o">.</span><span class="n">compose</span><span class="p">()</span>
<span class="go">[Unit("lm Wb")]</span>
</pre></div>
</div>
</div>
<div class="section" id="converting-between-systems">
<h2>Converting between systems<a class="headerlink" href="#converting-between-systems" title="Permalink to this headline">¶</a></h2>
<p>Built on top of this functionality is a convenience method to convert
between unit systems.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Pa</span><span class="o">.</span><span class="n">to_system</span><span class="p">(</span><span class="n">u</span><span class="o">.</span><span class="n">cgs</span><span class="p">)</span>
<span class="go">[Unit("1.000000e+01 Ba")]</span>
</pre></div>
</div>
<p>This is equivalent to decomposing into the new system and then
composing into the most complex units possible, though <tt class="xref py py-obj docutils literal"><span class="pre">to_system</span></tt>
adds some extra logic to return the results sorted in the most useful
order:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">u</span><span class="o">.</span><span class="n">Pa</span><span class="o">.</span><span class="n">decompose</span><span class="p">(</span><span class="n">bases</span><span class="o">=</span><span class="n">u</span><span class="o">.</span><span class="n">cgs</span><span class="o">.</span><span class="n">bases</span><span class="p">)</span>
<span class="go">Unit("1.000000e+01 g / (cm s2)")</span>
<span class="gp">>>> </span><span class="n">_</span><span class="o">.</span><span class="n">compose</span><span class="p">(</span><span class="n">units</span><span class="o">=</span><span class="n">u</span><span class="o">.</span><span class="n">cgs</span><span class="p">)</span>
<span class="go">[Unit("1.000000e+01 Ba")]</span>
</pre></div>
</div>
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<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper"><h3>Page Contents</h3>
<ul>
<li><a class="reference internal" href="#">Decomposing and composing units</a><ul>
<li><a class="reference internal" href="#reducing-a-unit-to-its-irreducible-parts">Reducing a unit to its irreducible parts</a></li>
<li><a class="reference internal" href="#automatically-composing-a-unit-into-more-complex-units">Automatically composing a unit into more complex units</a></li>
<li><a class="reference internal" href="#converting-between-systems">Converting between systems</a></li>
</ul>
</li>
</ul>
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