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whats-new, more special-method-names, typography fiddling
This commit is contained in:
Mark Pilgrim
+23
-23
@@ -17,7 +17,7 @@ body{counter-reset:h1 2}
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</blockquote>
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<p id=toc>
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<h2 id=divingin>Diving In</h2>
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<p class=f>Cast aside <a href=your-first-python-program.html>your first Python program</a> for just a minute, and let's talk about datatypes. In Python, <a href=your-first-python-program.html#datatypes>every variable has a datatype</a>, but you don't need to declare it explicitly. Based on each variable's original assignment, Python figures out what type it is and keeps tracks of that internally.
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<p class=f>Cast aside <a href=your-first-python-program.html>your first Python program</a> for just a minute, and let’s talk about datatypes. In Python, <a href=your-first-python-program.html#datatypes>every variable has a datatype</a>, but you don’t need to declare it explicitly. Based on each variable’s original assignment, Python figures out what type it is and keeps tracks of that internally.
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<p>Python has many native datatypes. Here are the important ones:
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<ol>
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<li><b>Booleans</b> are either <code>True</code> or <code>False</code>.
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@@ -28,8 +28,8 @@ body{counter-reset:h1 2}
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<li><b>Sets</b> are unordered bags of values.
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<li><b>Dictionaries</b> are unordered bags of key-value pairs.
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</ol>
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<p>Of course, there are a lot more types than these seven. <a href=your-first-python-program.html#everythingisanobject>Everything is an object</a> in Python, so there are types like <i>module</i>, <i>function</i>, <i>class</i>, <i>method</i>, <i>file</i>, and even <i>compiled code</i>. You've already seen some of these: <a href=your-first-python-program.html#runningscripts>modules have names</a>, <a href=your-first-python-program.html#docstrings>functions have <code>docstrings</code></a>, <i class=baa>&</i>c. You'll learn about classes in [FIXME xref] and files in [FIXME xref].
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<p>Strings and bytes are important enough — and complicated enough — that they get their own chapter. Let's look at the others first.
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<p>Of course, there are a lot more types than these seven. <a href=your-first-python-program.html#everythingisanobject>Everything is an object</a> in Python, so there are types like <i>module</i>, <i>function</i>, <i>class</i>, <i>method</i>, <i>file</i>, and even <i>compiled code</i>. You’ve already seen some of these: <a href=your-first-python-program.html#runningscripts>modules have names</a>, <a href=your-first-python-program.html#docstrings>functions have <code>docstrings</code></a>, <i class=baa>&</i>c. You’ll learn about classes in [FIXME xref] and files in [FIXME xref].
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<p>Strings and bytes are important enough — and complicated enough — that they get their own chapter. Let’s look at the others first.
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<h2 id=booleans>Booleans</h2>
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<aside>You can use virtually any expression in a boolean context.</aside>
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<p>Booleans are either true or false. Python has two constants, <code>True</code> and <code>False</code>, which can be used to assign boolean values directly. Expressions can also evaluate to a boolean value. In certain places (like <code>if</code> statements), Python expects an expression to evaluate to a boolean value. These places are called <i>boolean contexts</i>. You can use virtually any expression in a boolean context, and Python will try to determine its truth value. Different datatypes have different rules about which values are true or false in a boolean context. (This will make more sense once you see some concrete examples later in this chapter.)
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@@ -48,7 +48,7 @@ body{counter-reset:h1 2}
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<samp class=p>>>> </samp><kbd>size < 0</kbd>
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<samp>True</samp></pre>
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<h2 id=numbers>Numbers</h2>
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<p>Numbers are awesome. There are so many to choose from. Python supports both integers and floating point numbers. There's no type declaration to distinguish them; Python tells them apart by the presence or absence of a decimal point.
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<p>Numbers are awesome. There are so many to choose from. Python supports both integers and floating point numbers. There’s no type declaration to distinguish them; Python tells them apart by the presence or absence of a decimal point.
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<pre class=screen>
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<a><samp class=p>>>> </samp><kbd>type(1)</kbd> <span>①</span></a>
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<samp><class 'int'></samp>
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@@ -82,7 +82,7 @@ body{counter-reset:h1 2}
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<li>You can explicitly coerce an <code>int</code> to a <code>float</code> by calling the <code>float()</code> function.
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<li>Unsurprisingly, you can also coerce a <code>float</code> to an <code>int</code> by calling <code>int()</code>.
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<li>The <code>int()</code> function will truncate, not round.
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<li>The <code>int()</code> function truncates negative numbers towards <code>0</code>. It's a true truncate function, not a a floor function.
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<li>The <code>int()</code> function truncates negative numbers towards <code>0</code>. It’s a true truncate function, not a a floor function.
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<li>Floating point numbers are accurate to 15 decimal places.
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<li>Integers can be arbitrarily large.
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</ol>
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@@ -108,8 +108,8 @@ body{counter-reset:h1 2}
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<ol>
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<li>The <code>/</code> operator performs floating point division. It returns a <code>float</code> even if both the numerator and denominator are <code>int</code>s.
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<li>The <code>//</code> operator performs a quirky kind of integer division. When the result is positive, you can think of it as truncating (not rounding) to <code>0</code> decimal places, but be careful with that.
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<li>When integer-dividing negative numbers, the <code>//</code> operator rounds “up” to the nearest integer. Mathematically speaking, it's rounding “down” since <code>−6</code> is less than <code>−5</code>, but it could trip you up if you expecting it to truncate to <code>−5</code>.
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<li>The <code>//</code> operator doesn't always return an integer. If either the numerator or denominator is a <code>float</code>, it will still round to the nearest integer, but the actual return value will be a <code>float</code>.
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<li>When integer-dividing negative numbers, the <code>//</code> operator rounds “up” to the nearest integer. Mathematically speaking, it’s rounding “down” since <code>−6</code> is less than <code>−5</code>, but it could trip you up if you expecting it to truncate to <code>−5</code>.
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<li>The <code>//</code> operator doesn’t always return an integer. If either the numerator or denominator is a <code>float</code>, it will still round to the nearest integer, but the actual return value will be a <code>float</code>.
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<li>The <code>**</code> operator means “raised to the power of.” <code>11<sup>2</sup></code> is <code>121</code>.
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<li>The <code>%</code> operator gives the remainder after performing integer division. <code>11</code> divided by <code>2</code> is <code>5</code> with a remainder of <code>1</code>, so the result here is <code>1</code>.
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</ol>
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@@ -117,7 +117,7 @@ body{counter-reset:h1 2}
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<p><span>☞</span>In Python 2, the <code>/</code> operator usually meant integer division, but you could make it behave like floating point division by including a special directive in your code. In Python 3, the <code>/</code> operator always means floating point division. See <a href=http://www.python.org/dev/peps/pep-0238/><abbr>PEP</abbr> 238</a> for details.
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</blockquote>
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<h3 id=fractions>Fractions</h3>
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<p>Python isn't limited to integers and floating point numbers. It can also do all the fancy math you learned in high school and promptly forgot about.
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<p>Python isn’t limited to integers and floating point numbers. It can also do all the fancy math you learned in high school and promptly forgot about.
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<pre class=screen>
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<a><samp class=p>>>> </samp><kbd>import fractions</kbd> <span>①</span></a>
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<a><samp class=p>>>> </samp><kbd>x = fractions.Fraction(1, 3)</kbd> <span>②</span></a>
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@@ -144,7 +144,7 @@ body{counter-reset:h1 2}
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<a><samp class=p>>>> </samp><kbd>math.tan(math.pi / 4)</kbd> <span>③</span></a>
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<samp>0.99999999999999989</samp></pre>
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<ol>
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<li>The <code>math</code> module has a constant for π, the ratio of a circle's circumference to its diameter.
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<li>The <code>math</code> module has a constant for π, the ratio of a circle’s circumference to its diameter.
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<li>The <code>math</code> module has all the basic trigonometric functions, including <code>sin()</code>, <code>cos()</code>, <code>tan()</code>, and variants like <code>asin()</code>.
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<li>Note, however, that Python does not have infinite precision. <code>tan(π / 4)</code> should return <code>1.0</code>, not <code>0.99999999999999989</code>.
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</ol>
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@@ -176,16 +176,16 @@ body{counter-reset:h1 2}
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<ol>
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<li>Did you know you can define your own functions in the Python interactive shell? Just press <kbd>ENTER</kbd> at the end of each line, and <kbd>ENTER</kbd> on a blank line to finish.
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<li>In a boolean context, non-zero integers are true; <code>0</code> is false.
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<li>Non-zero floating point numbers are true; <code>0.0</code> is false. Be careful with this one! If there's the slightest rounding error (not impossible, as you saw in the previous section) then Python will be testing <code>0.0000000000001</code> instead of <code>0</code> and will return <code>True</code>.
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<li>Non-zero floating point numbers are true; <code>0.0</code> is false. Be careful with this one! If there’s the slightest rounding error (not impossible, as you saw in the previous section) then Python will be testing <code>0.0000000000001</code> instead of <code>0</code> and will return <code>True</code>.
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<li>Fractions can also be used in a boolean context. <code>Fraction(0, n)</code> is false for all values of <var>n</var>. All other fractions are true.
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</ol>
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<h2 id=lists>Lists</h2>
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<p>Lists are Python's workhorse datatype. When I say “list,” you might be thinking “array whose size I have to declare in advance, that can only contain items of the same type, <i class=baa>&</i>c.” Don't think that. Lists are much cooler than that.
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<p>Lists are Python’s workhorse datatype. When I say “list,” you might be thinking “array whose size I have to declare in advance, that can only contain items of the same type, <i class=baa>&</i>c.” Don’t think that. Lists are much cooler than that.
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<blockquote class="note compare perl5">
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<p><span>☞</span>A list in Python is like an array in Perl 5. In Perl 5, variables that store arrays always start with the <code>@</code> character; in Python, variables can be named anything, and Python keeps track of the datatype internally.
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</blockquote>
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<blockquote class="note compare java">
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<p><span>☞</span>A list in Python is much more than an array in Java (although it can be used as one if that's really all you want out of life). A better analogy would be to the <code>ArrayList</code> class, which can hold arbitrary objects and can expand dynamically as new items are added.
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<p><span>☞</span>A list in Python is much more than an array in Java (although it can be used as one if that’s really all you want out of life). A better analogy would be to the <code>ArrayList</code> class, which can hold arbitrary objects and can expand dynamically as new items are added.
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</blockquote>
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<h3 id=creatinglists>Creating A List</h3>
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<p>Creating a list is easy: use square brackets to wrap a comma-separated list of values.
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@@ -210,7 +210,7 @@ body{counter-reset:h1 2}
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</ol>
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<h3 id=slicinglists>Slicing A List</h3>
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<aside>a_list[0] is the first item of a_list.</aside>
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<p>Once you've defined a list, you can get any part of it as a new list. This is called <i>slicing</i> the list.
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<p>Once you’ve defined a list, you can get any part of it as a new list. This is called <i>slicing</i> the list.
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<pre class=screen>
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<samp class=p>>>> </samp><kbd>a_list</kbd>
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<samp>['a', 'b', 'mpilgrim', 'z', 'example']</samp>
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@@ -228,7 +228,7 @@ body{counter-reset:h1 2}
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['a', 'b', 'mpilgrim', 'z', 'example']</pre>
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<ol>
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<li>You can get a part of a list, called a “slice”, by specifying two indices. The return value is a new list containing all the items of the list, in order, starting with the first slice index (in this case <code>a_list[1]</code>), up to but not including the second slice index (in this case <code>a_list[3]</code>).
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<li>Slicing works if one or both of the slice indices is negative. If it helps, you can think of it this way: reading the list from left to right, the first slice index specifies the first item you want, and the second slice index specifies the first item you don't want. The return value is everything in between.
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<li>Slicing works if one or both of the slice indices is negative. If it helps, you can think of it this way: reading the list from left to right, the first slice index specifies the first item you want, and the second slice index specifies the first item you don’t want. The return value is everything in between.
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<li>Lists are zero-based, so <code>a_list[0:3]</code> returns the first three items of the list, starting at <code>a_list[0]</code>, up to but not including <code>a_list[3]</code>.
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<li>If the left slice index is <code>0</code>, you can leave it out, and <code>0</code> is implied. So <code>a_list[:3]</code> is the same as <code>a_list[0:3]</code>, because the starting <code>0</code> is implied.
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<li>Similarly, if the right slice index is the length of the list, you can leave it out. So <code>a_list[3:]</code> is the same as <code>a_list[3:5]</code>, because this list has five items. There is a pleasing symmetry here. In this five-item list, <code>a_list[:3]</code> returns the first 3 items, and <code>a_list[3:]</code> returns the last two items. In fact, <code>a_list[:<var>n</var>]</code> will always return the first <var>n</var> items, and <code>a_list[<var>n</var>:]</code> will return the rest, regardless of the length of the list.
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@@ -251,12 +251,12 @@ body{counter-reset:h1 2}
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<samp class=p>>>> </samp><kbd>a_list</kbd>
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<samp>['a', 'a', 2.0, 3, True, 'four', 'e']</samp></pre>
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<ol>
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<li>The <code>+</code> operator concatenates lists. A list can contain any number of items; there is no size limit (other than available memory). A list can contain items of any datatype; they don't all need to be the same type. Here we have a list containing a string, a floating point number, and an integer.
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<li>The <code>+</code> operator concatenates lists. A list can contain any number of items; there is no size limit (other than available memory). A list can contain items of any datatype; they don’t all need to be the same type. Here we have a list containing a string, a floating point number, and an integer.
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<li>The <code>append()</code> method adds a single item to the end of the list. (Now we have <em>four</em> different datatypes in the list!)
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<li>Lists are implemented as classes. “Creating” a list is really instantiating a class. As such, a list has methods that operate on it. The <code>extend()</code> method takes one argument, a list, and appends each of the items of the argument to the original list.
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<li>The <code>insert()</code> method inserts a single item into a list. The first argument is the index of the first item in the list that will get bumped out of position. List items do not need to be unique; for example, there are now two separate items with the value <code>'a'</code>, <code>a_list[0]</code> and <code>a_list[1]</code>.
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</ol>
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<p>Let's look closer at the difference between <code>append()</code> and <code>extend()</code>.
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<p>Let’s look closer at the difference between <code>append()</code> and <code>extend()</code>.
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<pre class=screen>
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<samp class=p>>>> </samp><kbd>a_list = ['a', 'b', 'c']</kbd>
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<a><samp class=p>>>> </samp><kbd>a_list.extend(['d', 'e', 'f'])</kbd> <span>①</span></a>
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@@ -276,8 +276,8 @@ body{counter-reset:h1 2}
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<ol>
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<li>The <code>extend()</code> method takes a single argument, which is always a list, and adds each of the items of that list to <var>a_list</var>.
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<li>If you start with a list of three items and extend it with a list of another three items, you end up with a list of six items.
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<li>On the other hand, the <code>append()</code> method takes any number of arguments, each of which can be any datatype. Here, you're calling the <code>append()</code> method with a single argument, a list of three items.
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<li>If you start with a list of six items and append a list onto it, you end up with... a list of seven items. Why seven? Because the last item (which you just appended) <em>is itself a list</em>. Lists can contain any type of data, including other lists. That may be what you want, or it may not. But it's what you asked for, and it's what you got.
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<li>On the other hand, the <code>append()</code> method takes any number of arguments, each of which can be any datatype. Here, you’re calling the <code>append()</code> method with a single argument, a list of three items.
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<li>If you start with a list of six items and append a list onto it, you end up with... a list of seven items. Why seven? Because the last item (which you just appended) <em>is itself a list</em>. Lists can contain any type of data, including other lists. That may be what you want, or it may not. But it’s what you asked for, and it’s what you got.
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</ol>
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<h3 id=searchinglists>Searching For Values In A List</h3>
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<pre class=screen>
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@@ -324,7 +324,7 @@ ValueError: list.index(x): x not in list</samp></pre>
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<p>FIXME
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-->
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<h2 id=dictionaries>Dictionaries</h2>
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<p>One of Python's most important datatypes is the dictionary, which defines one-to-one relationships between keys and values.
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<p>One of Python’s most important datatypes is the dictionary, which defines one-to-one relationships between keys and values.
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<blockquote class="note compare perl5">
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<p><span>☞</span>A dictionary in Python is like a hash in Perl 5. In Perl 5, variables that store hashes always start with a <code>%</code> character. In Python, variables can be named anything, and Python keeps track of the datatype internally.
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</blockquote>
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@@ -346,7 +346,7 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
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<li>First, you create a new dictionary with two items and assign it to the variable <var>a_dict</var>. Each item is a key-value pair, and the whole set of items is enclosed in curly braces.
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<li><code>'server'</code> is a key, and its associated value, referenced by <code>a_dict["server"]</code>, is <code>'db.diveintopython3.org'</code>.
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<li><code>'database'</code> is a key, and its associated value, referenced by <code>a_dict["database"]</code>, is <code>'mysql'</code>.
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<li>You can get values by key, but you can't get keys by value. So <code>a_dict["server"]</code> is <code>'db.diveintopython3.org'</code>, but <code>a_dict["db.diveintopython3.org"]</code> raises an exception, because <code>'db.diveintopython3.org'</code> is not a key.
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<li>You can get values by key, but you can’t get keys by value. So <code>a_dict["server"]</code> is <code>'db.diveintopython3.org'</code>, but <code>a_dict["db.diveintopython3.org"]</code> raises an exception, because <code>'db.diveintopython3.org'</code> is not a key.
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</ol>
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<h3 id=modifying-dictionaries>Modifying A Dictionary</h3>
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<p>Dictionaries do not have any predefined size limit. You can add new key-value pairs to a dictionary at any time, or you can modify the value of an existing key. Continuing from the previous example:
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@@ -370,11 +370,11 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
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<li>You can add new key-value pairs at any time. This syntax is identical to modifying existing values.
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<li>The new dictionary item (key <code>'user'</code>, value <code>'mark'</code>) appears to be in the middle. In fact, it was just a coincidence that the items appeared to be in order in the first example; it is just as much a coincidence that they appear to be out of order now.
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<li>Assigning a value to an existing dictionary key simply replaces the old value with the new one.
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<li>Will this change the value of the <code>user</code> key back to "mark"? No! Look at the key closely — that's a capital <kbd>U</kbd> in <kbd>"User"</kbd>. Dictionary keys are case-sensitive, so this statement is creating a new key-value pair, not overwriting an existing one. It may look similar to you, but as far as Python is concerned, it's completely different.
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<li>Will this change the value of the <code>user</code> key back to "mark"? No! Look at the key closely — that’s a capital <kbd>U</kbd> in <kbd>"User"</kbd>. Dictionary keys are case-sensitive, so this statement is creating a new key-value pair, not overwriting an existing one. It may look similar to you, but as far as Python is concerned, it’s completely different.
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</ol>
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<h3 id=mixed-value-dictionaries>Mixed-Value Dictionaries</h3>
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<p>Dictionaries aren't just for strings. Dictionary values can be any datatype, including integers, booleans, arbitrary objects, or even other dictionaries. And within a single dictionary, the values don't all need to be the same type; you can mix and match as needed. Dictionary keys are more restricted, but they can be strings, integers, and a few other types. You can also mix and match key datatypes within a dictionary.
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<p>In fact, you've already seen a dictionary with non-string keys and values, in <a href=your-first-python-program.html#divingin>your first Python program</a>.
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<p>Dictionaries aren’t just for strings. Dictionary values can be any datatype, including integers, booleans, arbitrary objects, or even other dictionaries. And within a single dictionary, the values don’t all need to be the same type; you can mix and match as needed. Dictionary keys are more restricted, but they can be strings, integers, and a few other types. You can also mix and match key datatypes within a dictionary.
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<p>In fact, you’ve already seen a dictionary with non-string keys and values, in <a href=your-first-python-program.html#divingin>your first Python program</a>.
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<pre><code>SUFFIXES = {1000: ('KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'),
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1024: ('KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB')}</code></pre>
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<p>Let's tear that apart in the interactive shell.
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