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https://github.com/kennethreitz/dive-into-python3.git
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moved #importsearchpath up, fixed directory references, plus a few typos
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Mark Pilgrim
@@ -103,7 +103,7 @@ if __name__ == '__main__':
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<a> print(approximate_size(1000000000000, False)) <span class=u>①</span></a>
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<a> print(approximate_size(1000000000000)) <span class=u>②</span></a></code></pre>
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<ol>
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<li>This calls the <code>approximate_size()</code> function with two argument. Within the <code>approximate_size()</code> function, <var>a_kilobyte_is_1024_bytes</var> will be <code>False</code>, since you explicitly passed <code>False</code> as the second argument.
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<li>This calls the <code>approximate_size()</code> function with two arguments. Within the <code>approximate_size()</code> function, <var>a_kilobyte_is_1024_bytes</var> will be <code>False</code>, since you explicitly passed <code>False</code> as the second argument.
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<li>This calls the <code>approximate_size()</code> function with only one argument. But that’s OK, because the second argument is optional! Since the caller doesn’t specify, the second argument defaults to <code>True</code>, as defined by the function declaration.
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</ol>
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@@ -159,6 +159,38 @@ SyntaxError: non-keyword arg after keyword arg</samp></pre>
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</blockquote>
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<p class=a>⁂
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<h2 id=importsearchpath>The <code>import</code> Search Path</h2>
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<p>Before this goes any further, I want to briefly mention the library search path. Python looks in several places when you try to import a module. Specifically, it looks in all the directories defined in <code>sys.path</code>. This is just a list, and you can easily view it or modify it with standard list methods. (You’ll learn more about lists in <a href=native-datatypes.html#lists>Native Datatypes</a>.)
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<pre class=screen>
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<a><samp class=p>>>> </samp><kbd class=pp>import sys</kbd> <span class=u>①</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path</kbd> <span class=u>②</span></a>
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<samp class=pp>['',
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'/usr/lib/python31.zip',
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'/usr/lib/python3.1',
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'/usr/lib/python3.1/plat-linux2@EXTRAMACHDEPPATH@',
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'/usr/lib/python3.1/lib-dynload',
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'/usr/lib/python3.1/dist-packages',
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'/usr/local/lib/python3.1/dist-packages']</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>sys</kbd> <span class=u>③</span></a>
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<samp class=pp><module 'sys' (built-in)></samp>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path.insert(0, '/home/mark/diveintopython3/examples')</kbd> <span class=u>④</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path</kbd> <span class=u>⑤</span></a>
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<samp class=pp>['/home/mark/py',
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'',
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'/usr/lib/python30.zip',
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'/usr/lib/python3.0',
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'/usr/lib/python3.0/plat-linux2@EXTRAMACHDEPPATH@',
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'/usr/lib/python3.0/lib-dynload',
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'/usr/lib/python3.0/dist-packages',
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'/usr/local/lib/python3.0/dist-packages']</samp></pre>
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<ol>
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<li>Importing the <code>sys</code> module makes all of its functions and attributes available.
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<li><code>sys.path</code> is a list of directory names that constitute the current search path. (Yours will look different, depending on your operating system, what version of Python you’re running, and where it was originally installed.) Python will look through these directories (in this order) for a <code>.py</code> file whose name matches what you’re trying to import.
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<li>Actually, I lied; the truth is more complicated than that, because not all modules are stored as <code>.py</code> files. Some, like the <code>sys</code> module, are <i>built-in modules</i>; they are actually baked right into Python itself. Built-in modules behave just like regular modules, but their Python source code is not available, because they are not written in Python! (The <code>sys</code> module is written in <abbr>C</abbr>.)
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<li>You can add a new directory to Python’s search path at runtime by adding the directory name to <code>sys.path</code>, and then Python will look in that directory as well, whenever you try to import a module. The effect lasts as long as Python is running.
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<li>By using <code>sys.path.insert(0, <var>new_path</var>)</code>, you inserted a new directory as the first item of the <code>sys.path</code> list, and therefore at the beginning of Python’s search path. This is almost always what you want. In case of naming conflicts (for example, if Python ships with version 2 of a particular library but you want to use version 3), this ensures that your modules will be found and used instead of the modules that came with Python.
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</ol>
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<h2 id=everythingisanobject>Everything Is An Object</h2>
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<p>In case you missed it, I just said that Python functions have attributes, and that those attributes are available at runtime. A function, like everything else in Python, is an object.
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<p>Run the interactive Python shell and follow along:
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@@ -185,37 +217,6 @@ SyntaxError: non-keyword arg after keyword arg</samp></pre>
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<blockquote class='note compare perl5'>
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<p><span class=u>☞</span><code>import</code> in Python is like <code>require</code> in Perl. Once you <code>import</code> a Python module, you access its functions with <code><var>module</var>.<var>function</var></code>; once you <code>require</code> a Perl module, you access its functions with <code><var>module</var>::<var>function</var></code>.
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</blockquote>
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<h3 id=importsearchpath>The <code>import</code> Search Path</h3>
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<p>Before this goes any further, I want to briefly mention the library search path. Python looks in several places when you try to import a module. Specifically, it looks in all the directories defined in <code>sys.path</code>. This is just a list, and you can easily view it or modify it with standard list methods. (You’ll learn more about lists in <a href=native-datatypes.html#lists>Native Datatypes</a>.)
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<pre class=screen>
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<a><samp class=p>>>> </samp><kbd class=pp>import sys</kbd> <span class=u>①</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path</kbd> <span class=u>②</span></a>
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<samp class=pp>['',
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'/usr/lib/python30.zip',
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'/usr/lib/python3.0',
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'/usr/lib/python3.0/plat-linux2@EXTRAMACHDEPPATH@',
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'/usr/lib/python3.0/lib-dynload',
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'/usr/lib/python3.0/dist-packages',
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'/usr/local/lib/python3.0/dist-packages']</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>sys</kbd> <span class=u>③</span></a>
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<samp class=pp><module 'sys' (built-in)></samp>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path.insert(0, '/home/mark/py')</kbd> <span class=u>④</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>sys.path</kbd> <span class=u>⑤</span></a>
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<samp class=pp>['/home/mark/py',
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'',
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'/usr/lib/python30.zip',
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'/usr/lib/python3.0',
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'/usr/lib/python3.0/plat-linux2@EXTRAMACHDEPPATH@',
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'/usr/lib/python3.0/lib-dynload',
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'/usr/lib/python3.0/dist-packages',
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'/usr/local/lib/python3.0/dist-packages']</samp></pre>
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<ol>
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<li>Importing the <code>sys</code> module makes all of its functions and attributes available.
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<li><code>sys.path</code> is a list of directory names that constitute the current search path. (Yours will look different, depending on your operating system, what version of Python you’re running, and where it was originally installed.) Python will look through these directories (in this order) for a <code>.py</code> file whose name matches what you’re trying to import.
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<li>Actually, I lied; the truth is more complicated than that, because not all modules are stored as <code>.py</code> files. Some, like the <code>sys</code> module, are <i>built-in modules</i>; they are actually baked right into Python itself. Built-in modules behave just like regular modules, but their Python source code is not available, because they are not written in Python! (The <code>sys</code> module is written in <abbr>C</abbr>.)
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<li>You can add a new directory to Python’s search path at runtime by adding the directory name to <code>sys.path</code>, and then Python will look in that directory as well, whenever you try to import a module. The effect lasts as long as Python is running.
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<li>By using <code>sys.path.insert(0, <var>new_path</var>)</code>, you inserted a new directory as the first item of the <code>sys.path</code> list, and therefore at the beginning of Python’s search path. This is almost always what you want. In case of naming conflicts (for example, if Python ships with version 2 of a particular library but you want to use version 3), this ensures that your modules will be found and used instead of the modules that came with Python.
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</ol>
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<h3 id=whatsanobject>What’s An Object?</h3>
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<p>Everything in Python is an object, and everything can have attributes and methods. All functions have a built-in attribute <code>__doc__</code>, which returns the <var>docstring</var> defined in the function’s source code. The <code>sys</code> module is an object which has (among other things) an attribute called <var>path</var>. And so forth.
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<p>Still, this doesn’t answer the more fundamental question: what is an object? Different programming languages define “object” in different ways. In some, it means that <em>all</em> objects <em>must</em> have attributes and methods; in others, it means that all objects are subclassable. In Python, the definition is looser. Some objects have neither attributes nor methods, <em>but they could</em>. Not all objects are subclassable. But everything is an object in the sense that it can be assigned to a variable or passed as an argument to a function.
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