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rougier committed Dec 31, 2016
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45 changes: 36 additions & 9 deletions 03-anatomy.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -333,20 +333,54 @@ copy:
>>> print(Z2.base is None)
True
Note that some numpy functions return a view while some others return a copy:
Note that some numpy functions return a view when possible (e.g. `ravel
<https://docs.scipy.org/doc/numpy/reference/generated/numpy.ravel.html>`_)
while some others always return a copy (e.g. `flatten
<https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.flatten.html#numpy.ndarray.flatten>`_):
.. code:: pycon
>>> Z = np.arange(9).reshape(3,3).copy()
>>> Z = np.zeros((5,5))
>>> Z.ravel().base is Z
True
>>> Z[::2,::2].ravel().base is Z
False
>>> Z.flatten().base is Z
False
Temporary copy
++++++++++++++
Copies can be made explicitly like in the previous section, but the most
general case is the implicit creation of intermediate copies. This is the case
when you are doing some arithmetic with arrays:
.. code:: pycon
>>> X = np.ones(10, dtype=np.int)
>>> Y = np.ones(10, dtype=np.int)
>>> A = 2*X + 2*Y
In the example above, three intermediate arrays have been created. One for
holding the result of `2*X`, one for holding the result of `2*Y` and the last
one for holding the result of `2*X+2*Y`. In this specific case, the arrays are
small enough and this does not really make a difference. However, if your
arrays are big, then you have be careful with such expression and wonder if you
can do it differently. For example, if only the final result matters and you
don't need `X` nor `Y` afterwards, an alternate solution would be:
.. code:: pycon
>>> X = np.ones(10, dtype=np.int)
>>> Y = np.ones(10, dtype=np.int)
>>> np.multiply(X, 2, out=X)
>>> np.multiply(Y, 2, out=Y)
>>> np.add(X, Y, out=X)
Using this alternate solution, no temporary array has been created. Problem is
that there are many other cases where such copies needs to be created and this
impact the performance like demonstrated on the example below:
.. code:: pycon
Expand All @@ -362,13 +396,6 @@ Temporary copy
1000 loops, best of 3: 1.57 ms per loop
Regular indexing returns a view
Conclusion
----------
Expand Down
33 changes: 30 additions & 3 deletions book.html
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Expand Up @@ -674,17 +674,45 @@ <h3><a class="toc-backref" href="#id2">Direct and indirect access</a></h3>
</span><span class="keyword"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="keyword">print</span><span class="punctuation">(</span><span class="name">Z2</span><span class="operator">.</span><span class="name">base</span> <span class="operator word">is</span> <span class="name builtin pseudo">None</span><span class="punctuation">)</span>
<span class="generic output">True</span>
</pre>
<p>Note that some numpy functions return a view while some others return a copy:</p>
<p>Note that some numpy functions return a view when possible (e.g. <a class="reference external" href="https://docs.scipy.org/doc/numpy/reference/generated/numpy.ravel.html">ravel</a>)
while some others always return a copy (e.g. <a class="reference external" href="https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.flatten.html#numpy.ndarray.flatten">flatten</a>):</p>
<pre class="code pycon literal-block">
<span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Z</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">arange</span><span class="punctuation">(</span><span class="literal number integer">9</span><span class="punctuation">)</span><span class="operator">.</span><span class="name">reshape</span><span class="punctuation">(</span><span class="literal number integer">3</span><span class="punctuation">,</span><span class="literal number integer">3</span><span class="punctuation">)</span><span class="operator">.</span><span class="name">copy</span><span class="punctuation">()</span>
<span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Z</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">zeros</span><span class="punctuation">((</span><span class="literal number integer">5</span><span class="punctuation">,</span><span class="literal number integer">5</span><span class="punctuation">))</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Z</span><span class="operator">.</span><span class="name">ravel</span><span class="punctuation">()</span><span class="operator">.</span><span class="name">base</span> <span class="operator word">is</span> <span class="name">Z</span>
<span class="generic output">True
</span><span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Z</span><span class="punctuation">[::</span><span class="literal number integer">2</span><span class="punctuation">,::</span><span class="literal number integer">2</span><span class="punctuation">]</span><span class="operator">.</span><span class="name">ravel</span><span class="punctuation">()</span><span class="operator">.</span><span class="name">base</span> <span class="operator word">is</span> <span class="name">Z</span>
<span class="generic output">False
</span><span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Z</span><span class="operator">.</span><span class="name">flatten</span><span class="punctuation">()</span><span class="operator">.</span><span class="name">base</span> <span class="operator word">is</span> <span class="name">Z</span>
<span class="generic output">False</span>
</pre>
</div>
<div class="section" id="temporary-copy">
<h3><a class="toc-backref" href="#id2">Temporary copy</a></h3>
<p>Copies can be made explicitly like in the previous section, but the most
general case is the implicit creation of intermediate copies. This is the case
when you are doing some arithmetic with arrays:</p>
<pre class="code pycon literal-block">
<span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">X</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">10</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Y</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">10</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">A</span> <span class="operator">=</span> <span class="literal number integer">2</span><span class="operator">*</span><span class="name">X</span> <span class="operator">+</span> <span class="literal number integer">2</span><span class="operator">*</span><span class="name">Y</span>
</pre>
<p>In the example above, three intermediate arrays have been created. One for
holding the result of <code>2*X</code>, one for holding the result of <code>2*Y</code> and the last
one for holding the result of <code>2*X+2*Y</code>. In this specific case, the arrays are
small enough and this does not really make a difference. However, if your
arrays are big, then you have be careful with such expression and wonder if you
can do it differently. For example, if only the final result matters and you
don't need <code>X</code> nor <code>Y</code> afterwards, an alternate solution would be:</p>
<pre class="code pycon literal-block">
<span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">X</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">10</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Y</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">10</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">np</span><span class="operator">.</span><span class="name">multiply</span><span class="punctuation">(</span><span class="name">X</span><span class="punctuation">,</span> <span class="literal number integer">2</span><span class="punctuation">,</span> <span class="name">out</span><span class="operator">=</span><span class="name">X</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">np</span><span class="operator">.</span><span class="name">multiply</span><span class="punctuation">(</span><span class="name">Y</span><span class="punctuation">,</span> <span class="literal number integer">2</span><span class="punctuation">,</span> <span class="name">out</span><span class="operator">=</span><span class="name">Y</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">np</span><span class="operator">.</span><span class="name">add</span><span class="punctuation">(</span><span class="name">X</span><span class="punctuation">,</span> <span class="name">Y</span><span class="punctuation">,</span> <span class="name">out</span><span class="operator">=</span><span class="name">X</span><span class="punctuation">)</span>
</pre>
<p>Using this alternate solution, no temporary array has been created. Problem is
that there are many other cases where such copies needs to be created and this
impact the performance like demonstrated on the example below:</p>
<pre class="code pycon literal-block">
<span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">X</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">1000000000</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
<span class="generic prompt">&gt;&gt;&gt; </span><span class="name">Y</span> <span class="operator">=</span> <span class="name">np</span><span class="operator">.</span><span class="name">ones</span><span class="punctuation">(</span><span class="literal number integer">1000000000</span><span class="punctuation">,</span> <span class="name">dtype</span><span class="operator">=</span><span class="name">np</span><span class="operator">.</span><span class="name">int</span><span class="punctuation">)</span>
Expand All @@ -697,7 +725,6 @@ <h3><a class="toc-backref" href="#id2">Temporary copy</a></h3>
</span><span class="name"></span><span class="generic prompt">&gt;&gt;&gt; </span><span class="name">np</span><span class="operator">.</span><span class="name">add</span><span class="punctuation">(</span><span class="name">X</span><span class="punctuation">,</span> <span class="name">Y</span><span class="punctuation">,</span> <span class="name">out</span><span class="operator">=</span><span class="name">X</span><span class="punctuation">),</span> <span class="name">np</span><span class="operator">.</span><span class="name">add</span><span class="punctuation">(</span><span class="name">X</span><span class="punctuation">,</span> <span class="name">Y</span><span class="punctuation">,</span> <span class="name">out</span><span class="operator">=</span><span class="name">X</span><span class="punctuation">),</span>
<span class="generic output">1000 loops, best of 3: 1.57 ms per loop</span>
</pre>
<p>Regular indexing returns a view</p>
</div>
</div>
<div class="section" id="conclusion">
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