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<div class="section" id="module-fractions">
<span id="fractions-rational-numbers"></span><h1>9.5. <a class="reference internal" href="#module-fractions" title="fractions: Rational numbers."><tt class="xref py py-mod docutils literal"><span class="pre">fractions</span></tt></a> — Rational numbers<a class="headerlink" href="#module-fractions" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/fractions.py">Lib/fractions.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-fractions" title="fractions: Rational numbers."><tt class="xref py py-mod docutils literal"><span class="pre">fractions</span></tt></a> module provides support for rational number arithmetic.</p>
<p>A Fraction instance can be constructed from a pair of integers, from
another rational number, or from a string.</p>
<dl class="class">
<dt id="fractions.Fraction">
<em class="property">class </em><tt class="descclassname">fractions.</tt><tt class="descname">Fraction</tt><big>(</big><em>numerator=0</em>, <em>denominator=1</em><big>)</big><a class="headerlink" href="#fractions.Fraction" title="Permalink to this definition">¶</a></dt>
<dt>
<em class="property">class </em><tt class="descclassname">fractions.</tt><tt class="descname">Fraction</tt><big>(</big><em>other_fraction</em><big>)</big></dt>
<dt>
<em class="property">class </em><tt class="descclassname">fractions.</tt><tt class="descname">Fraction</tt><big>(</big><em>float</em><big>)</big></dt>
<dt>
<em class="property">class </em><tt class="descclassname">fractions.</tt><tt class="descname">Fraction</tt><big>(</big><em>decimal</em><big>)</big></dt>
<dt>
<em class="property">class </em><tt class="descclassname">fractions.</tt><tt class="descname">Fraction</tt><big>(</big><em>string</em><big>)</big></dt>
<dd><p>The first version requires that <em>numerator</em> and <em>denominator</em> are instances
of <a class="reference internal" href="numbers.html#numbers.Rational" title="numbers.Rational"><tt class="xref py py-class docutils literal"><span class="pre">numbers.Rational</span></tt></a> and returns a new <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instance
with value <tt class="docutils literal"><span class="pre">numerator/denominator</span></tt>. If <em>denominator</em> is <tt class="xref py py-const docutils literal"><span class="pre">0</span></tt>, it
raises a <a class="reference internal" href="exceptions.html#exceptions.ZeroDivisionError" title="exceptions.ZeroDivisionError"><tt class="xref py py-exc docutils literal"><span class="pre">ZeroDivisionError</span></tt></a>. The second version requires that
<em>other_fraction</em> is an instance of <a class="reference internal" href="numbers.html#numbers.Rational" title="numbers.Rational"><tt class="xref py py-class docutils literal"><span class="pre">numbers.Rational</span></tt></a> and returns a
<a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instance with the same value. The next two versions accept
either a <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a> or a <a class="reference internal" href="decimal.html#decimal.Decimal" title="decimal.Decimal"><tt class="xref py py-class docutils literal"><span class="pre">decimal.Decimal</span></tt></a> instance, and return a
<a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instance with exactly the same value. Note that due to the
usual issues with binary floating-point (see <a class="reference internal" href="../tutorial/floatingpoint.html#tut-fp-issues"><em>Floating Point Arithmetic: Issues and Limitations</em></a>), the
argument to <tt class="docutils literal"><span class="pre">Fraction(1.1)</span></tt> is not exactly equal to 11/10, and so
<tt class="docutils literal"><span class="pre">Fraction(1.1)</span></tt> does <em>not</em> return <tt class="docutils literal"><span class="pre">Fraction(11,</span> <span class="pre">10)</span></tt> as one might expect.
(But see the documentation for the <a class="reference internal" href="#fractions.Fraction.limit_denominator" title="fractions.Fraction.limit_denominator"><tt class="xref py py-meth docutils literal"><span class="pre">limit_denominator()</span></tt></a> method below.)
The last version of the constructor expects a string or unicode instance.
The usual form for this instance is:</p>
<div class="highlight-python"><pre>[sign] numerator ['/' denominator]</pre>
</div>
<p>where the optional <tt class="docutils literal"><span class="pre">sign</span></tt> may be either ‘+’ or ‘-‘ and
<tt class="docutils literal"><span class="pre">numerator</span></tt> and <tt class="docutils literal"><span class="pre">denominator</span></tt> (if present) are strings of
decimal digits. In addition, any string that represents a finite
value and is accepted by the <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a> constructor is also
accepted by the <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> constructor. In either form the
input string may also have leading and/or trailing whitespace.
Here are some examples:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fractions</span> <span class="kn">import</span> <span class="n">Fraction</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="mi">16</span><span class="p">,</span> <span class="o">-</span><span class="mi">10</span><span class="p">)</span>
<span class="go">Fraction(-8, 5)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="mi">123</span><span class="p">)</span>
<span class="go">Fraction(123, 1)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">()</span>
<span class="go">Fraction(0, 1)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">'3/7'</span><span class="p">)</span>
<span class="go">Fraction(3, 7)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">' -3/7 '</span><span class="p">)</span>
<span class="go">Fraction(-3, 7)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">'1.414213 </span><span class="se">\t\n</span><span class="s">'</span><span class="p">)</span>
<span class="go">Fraction(1414213, 1000000)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">'-.125'</span><span class="p">)</span>
<span class="go">Fraction(-1, 8)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">'7e-6'</span><span class="p">)</span>
<span class="go">Fraction(7, 1000000)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="mf">2.25</span><span class="p">)</span>
<span class="go">Fraction(9, 4)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="mf">1.1</span><span class="p">)</span>
<span class="go">Fraction(2476979795053773, 2251799813685248)</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">decimal</span> <span class="kn">import</span> <span class="n">Decimal</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="n">Decimal</span><span class="p">(</span><span class="s">'1.1'</span><span class="p">))</span>
<span class="go">Fraction(11, 10)</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> class inherits from the abstract base class
<a class="reference internal" href="numbers.html#numbers.Rational" title="numbers.Rational"><tt class="xref py py-class docutils literal"><span class="pre">numbers.Rational</span></tt></a>, and implements all of the methods and
operations from that class. <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instances are hashable,
and should be treated as immutable. In addition,
<a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> has the following methods:</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 2.7: </span>The <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> constructor now accepts <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a> and
<a class="reference internal" href="decimal.html#decimal.Decimal" title="decimal.Decimal"><tt class="xref py py-class docutils literal"><span class="pre">decimal.Decimal</span></tt></a> instances.</p>
<dl class="method">
<dt id="fractions.Fraction.from_float">
<tt class="descname">from_float</tt><big>(</big><em>flt</em><big>)</big><a class="headerlink" href="#fractions.Fraction.from_float" title="Permalink to this definition">¶</a></dt>
<dd><p>This class method constructs a <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> representing the exact
value of <em>flt</em>, which must be a <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a>. Beware that
<tt class="docutils literal"><span class="pre">Fraction.from_float(0.3)</span></tt> is not the same value as <tt class="docutils literal"><span class="pre">Fraction(3,</span> <span class="pre">10)</span></tt></p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">From Python 2.7 onwards, you can also construct a
<a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instance directly from a <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a>.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="fractions.Fraction.from_decimal">
<tt class="descname">from_decimal</tt><big>(</big><em>dec</em><big>)</big><a class="headerlink" href="#fractions.Fraction.from_decimal" title="Permalink to this definition">¶</a></dt>
<dd><p>This class method constructs a <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> representing the exact
value of <em>dec</em>, which must be a <a class="reference internal" href="decimal.html#decimal.Decimal" title="decimal.Decimal"><tt class="xref py py-class docutils literal"><span class="pre">decimal.Decimal</span></tt></a>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">From Python 2.7 onwards, you can also construct a
<a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> instance directly from a <a class="reference internal" href="decimal.html#decimal.Decimal" title="decimal.Decimal"><tt class="xref py py-class docutils literal"><span class="pre">decimal.Decimal</span></tt></a>
instance.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="fractions.Fraction.limit_denominator">
<tt class="descname">limit_denominator</tt><big>(</big><em>max_denominator=1000000</em><big>)</big><a class="headerlink" href="#fractions.Fraction.limit_denominator" title="Permalink to this definition">¶</a></dt>
<dd><p>Finds and returns the closest <a class="reference internal" href="#fractions.Fraction" title="fractions.Fraction"><tt class="xref py py-class docutils literal"><span class="pre">Fraction</span></tt></a> to <tt class="docutils literal"><span class="pre">self</span></tt> that has
denominator at most max_denominator. This method is useful for finding
rational approximations to a given floating-point number:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fractions</span> <span class="kn">import</span> <span class="n">Fraction</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="s">'3.1415926535897932'</span><span class="p">)</span><span class="o">.</span><span class="n">limit_denominator</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="go">Fraction(355, 113)</span>
</pre></div>
</div>
<p>or for recovering a rational number that’s represented as a float:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">pi</span><span class="p">,</span> <span class="n">cos</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="n">cos</span><span class="p">(</span><span class="n">pi</span><span class="o">/</span><span class="mi">3</span><span class="p">))</span>
<span class="go">Fraction(4503599627370497, 9007199254740992)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="n">cos</span><span class="p">(</span><span class="n">pi</span><span class="o">/</span><span class="mi">3</span><span class="p">))</span><span class="o">.</span><span class="n">limit_denominator</span><span class="p">()</span>
<span class="go">Fraction(1, 2)</span>
<span class="gp">>>> </span><span class="n">Fraction</span><span class="p">(</span><span class="mf">1.1</span><span class="p">)</span><span class="o">.</span><span class="n">limit_denominator</span><span class="p">()</span>
<span class="go">Fraction(11, 10)</span>
</pre></div>
</div>
</dd></dl>
</dd></dl>
<dl class="function">
<dt id="fractions.gcd">
<tt class="descclassname">fractions.</tt><tt class="descname">gcd</tt><big>(</big><em>a</em>, <em>b</em><big>)</big><a class="headerlink" href="#fractions.gcd" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the greatest common divisor of the integers <em>a</em> and <em>b</em>. If either
<em>a</em> or <em>b</em> is nonzero, then the absolute value of <tt class="docutils literal"><span class="pre">gcd(a,</span> <span class="pre">b)</span></tt> is the
largest integer that divides both <em>a</em> and <em>b</em>. <tt class="docutils literal"><span class="pre">gcd(a,b)</span></tt> has the same
sign as <em>b</em> if <em>b</em> is nonzero; otherwise it takes the sign of <em>a</em>. <tt class="docutils literal"><span class="pre">gcd(0,</span>
<span class="pre">0)</span></tt> returns <tt class="docutils literal"><span class="pre">0</span></tt>.</p>
</dd></dl>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="numbers.html#module-numbers" title="numbers: Numeric abstract base classes (Complex, Real, Integral, etc.)."><tt class="xref py py-mod docutils literal"><span class="pre">numbers</span></tt></a></dt>
<dd>The abstract base classes making up the numeric tower.</dd>
</dl>
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