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mention 'first-class-objects' in your-first-python-program#whatsanobject

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Mark Pilgrim
2009-05-15 16:13:11 -04:00
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@@ -214,9 +214,10 @@ SyntaxError: non-keyword arg after keyword arg</samp></pre>
<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&#8217;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.
</ol>
<h3 id=whatsanobject>What&#8217;s An Object?</h3>
<p>Everything in Python is an object, and almost everything has attributes and methods. All functions have a built-in attribute <code>__doc__</code>, which returns the <var>docstring</var> defined in the function&#8217;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.
<p>Still, this doesn&#8217;t answer the more fundamental question: what is an object? Different programming languages define &#8220;object&#8221; 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 (more on this in [FIXME xref-was-#datatypes]), and not all objects are subclassable (more on this in [FIXME xref-was-#fileinfo]). But everything is an object in the sense that it can be assigned to a variable or passed as an argument to a function (more in this in [FIXME xref-was-#apihelp]).
<p>This is so important that I&#8217;m going to repeat it in case you missed it the first few times: <em>everything in Python is an object</em>. Strings are objects. Lists are objects. Functions are objects. Even modules are objects.
<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&#8217;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.
<p>Still, this doesn&#8217;t answer the more fundamental question: what is an object? Different programming languages define &#8220;object&#8221; 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.
<p>You may have heard the term &#8220;first-class object&#8221; in other programming contexts. In Python, functions are <i>first-class objects</i>. You can pass a function as an argument to another function. Modules are objects. You can pass an entire module as an argument to a function. Classes are objects, and individual instances of a class are also objects.
<p>This is important, so I&#8217;m going to repeat it in case you missed it the first few times: <em>everything in Python is an object</em>. Strings are objects. Lists are objects. Functions are objects. Classes are objects. Class instances are objects. Even modules are objects.
<h2 id=indentingcode>Indenting Code</h2>
<p>Python functions have no explicit <code>begin</code> or <code>end</code>, and no curly braces to mark where the function code starts and stops. The only delimiter is a colon (<code>:</code>) and the indentation of the code itself.
<pre><code>