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 <div class="section" id="input-and-output">
<h1>7. Input and Output<a class="headerlink" href="#input-and-output" title="Permalink to this headline">¶</a></h1>
There are several ways to present the output of a program; data can be printed
in a human-readable form, or written to a file for future use. This chapter will
discuss some of the possibilities.
<div class="section" id="fancier-output-formatting">
<h2>7.1. Fancier Output Formatting<a class="headerlink" href="#fancier-output-formatting" title="Permalink to this headline">¶</a></h2>
So far we&#8217;ve encountered two ways of writing values: expression statements and
the <a class="reference external" href="../reference/simple_stmts.html#print"><tt class="xref docutils literal">print</tt></a> statement. (A third way is using the <tt class="xref docutils literal">write()</tt> method
of file objects; the standard output file can be referenced as <tt class="docutils literal">sys.stdout</tt>.
See the Library Reference for more information on this.)
Often you&#8217;ll want more control over the formatting of your output than simply
printing space-separated values. There are two ways to format your output; the
first way is to do all the string handling yourself; using string slicing and
concatenation operations you can create any layout you can imagine. The
standard module <a title="Common string operations." class="reference external" href="../library/string.html#module-string"><tt class="xref docutils literal">string</tt></a> contains some useful operations for padding
strings to a given column width; these will be discussed shortly. The second
way is to use the <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">str.format()</tt></a> method.
One question remains, of course: how do you convert values to strings? Luckily,
Python has ways to convert any value to a string: pass it to the <a title="repr" class="reference external" href="../library/functions.html#repr"><tt class="xref docutils literal">repr()</tt></a>
or <a title="str" class="reference external" href="../library/functions.html#str"><tt class="xref docutils literal">str()</tt></a> functions.
The <a title="str" class="reference external" href="../library/functions.html#str"><tt class="xref docutils literal">str()</tt></a> function is meant to return representations of values which are
fairly human-readable, while <a title="repr" class="reference external" href="../library/functions.html#repr"><tt class="xref docutils literal">repr()</tt></a> is meant to generate representations
which can be read by the interpreter (or will force a <a title="exceptions.SyntaxError" class="reference external" href="../library/exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal">SyntaxError</tt></a> if
there is not equivalent syntax). For objects which don&#8217;t have a particular
representation for human consumption, <a title="str" class="reference external" href="../library/functions.html#str"><tt class="xref docutils literal">str()</tt></a> will return the same value as
<a title="repr" class="reference external" href="../library/functions.html#repr"><tt class="xref docutils literal">repr()</tt></a>. Many values, such as numbers or structures like lists and
dictionaries, have the same representation using either function. Strings and
floating point numbers, in particular, have two distinct representations.
Some examples:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = &#39;Hello, world.&#39;
&gt;&gt;&gt; str(s)
&#39;Hello, world.&#39;
&gt;&gt;&gt; repr(s)
&quot;&#39;Hello, world.&#39;&quot;
&gt;&gt;&gt; str(0.1)
&#39;0.1&#39;
&gt;&gt;&gt; repr(0.1)
&#39;0.10000000000000001&#39;
&gt;&gt;&gt; x = 10 * 3.25
&gt;&gt;&gt; y = 200 * 200
&gt;&gt;&gt; s = &#39;The value of x is &#39; + repr(x) + &#39;, and y is &#39; + repr(y) + &#39;...&#39;
&gt;&gt;&gt; print s
The value of x is 32.5, and y is 40000...
&gt;&gt;&gt; # The repr() of a string adds string quotes and backslashes:
... hello = &#39;hello, world\n&#39;
&gt;&gt;&gt; hellos = repr(hello)
&gt;&gt;&gt; print hellos
&#39;hello, world\n&#39;
&gt;&gt;&gt; # The argument to repr() may be any Python object:
... repr((x, y, (&#39;spam&#39;, &#39;eggs&#39;)))
&quot;(32.5, 40000, (&#39;spam&#39;, &#39;eggs&#39;))&quot;
</pre></div>
</div>
Here are two ways to write a table of squares and cubes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for x in range(1, 11):
... print repr(x).rjust(2), repr(x*x).rjust(3),
... # Note trailing comma on previous line
... print repr(x*x*x).rjust(4)
...
 1 1 1
 2 4 8
 3 9 27
 4 16 64
 5 25 125
 6 36 216
 7 49 343
 8 64 512
 9 81 729
10 100 1000

&gt;&gt;&gt; for x in range(1,11):
... print &#39;{0:2d} {1:3d} {2:4d}&#39;.format(x, x*x, x*x*x)
...
 1 1 1
 2 4 8
 3 9 27
 4 16 64
 5 25 125
 6 36 216
 7 49 343
 8 64 512
 9 81 729
10 100 1000
</pre></div>
</div>
(Note that in the first example, one space between each column was added by the
way <a class="reference external" href="../reference/simple_stmts.html#print"><tt class="xref docutils literal">print</tt></a> works: it always adds spaces between its arguments.)
This example demonstrates the <tt class="xref docutils literal">rjust()</tt> method of string objects, which
right-justifies a string in a field of a given width by padding it with spaces
on the left. There are similar methods <tt class="xref docutils literal">ljust()</tt> and <tt class="xref docutils literal">center()</tt>. These
methods do not write anything, they just return a new string. If the input
string is too long, they don&#8217;t truncate it, but return it unchanged; this will
mess up your column lay-out but that&#8217;s usually better than the alternative,
which would be lying about a value. (If you really want truncation you can
always add a slice operation, as in <tt class="docutils literal">x.ljust(n)[:n]</tt>.)
There is another method, <tt class="xref docutils literal">zfill()</tt>, which pads a numeric string on the left
with zeros. It understands about plus and minus signs:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; &#39;12&#39;.zfill(5)
&#39;00012&#39;
&gt;&gt;&gt; &#39;-3.14&#39;.zfill(7)
&#39;-003.14&#39;
&gt;&gt;&gt; &#39;3.14159265359&#39;.zfill(5)
&#39;3.14159265359&#39;
</pre></div>
</div>
Basic usage of the <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">str.format()</tt></a> method looks like this:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print &#39;We are the {0} who say &quot;{1}!&quot;&#39;.format(&#39;knights&#39;, &#39;Ni&#39;)
We are the knights who say &quot;Ni!&quot;
</pre></div>
</div>
The brackets and characters within them (called format fields) are replaced with
the objects passed into the <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">format()</tt></a> method. A number in the
brackets refers to the position of the object passed into the
<a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">format()</tt></a> method.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print &#39;{0} and {1}&#39;.format(&#39;spam&#39;, &#39;eggs&#39;)
spam and eggs
&gt;&gt;&gt; print &#39;{1} and {0}&#39;.format(&#39;spam&#39;, &#39;eggs&#39;)
eggs and spam
</pre></div>
</div>
If keyword arguments are used in the <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">format()</tt></a> method, their values
are referred to by using the name of the argument.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print &#39;This {food} is {adjective}.&#39;.format(
... food=&#39;spam&#39;, adjective=&#39;absolutely horrible&#39;)
This spam is absolutely horrible.
</pre></div>
</div>
Positional and keyword arguments can be arbitrarily combined:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print &#39;The story of {0}, {1}, and {other}.&#39;.format(&#39;Bill&#39;, &#39;Manfred&#39;,
... other=&#39;Georg&#39;)
The story of Bill, Manfred, and Georg.
</pre></div>
</div>
<tt class="docutils literal">'!s'</tt> (apply <a title="str" class="reference external" href="../library/functions.html#str"><tt class="xref docutils literal">str()</tt></a>) and <tt class="docutils literal">'!r'</tt> (apply <a title="repr" class="reference external" href="../library/functions.html#repr"><tt class="xref docutils literal">repr()</tt></a>) can be used to
convert the value before it is formatted.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import math
&gt;&gt;&gt; print &#39;The value of PI is approximately {0}.&#39;.format(math.pi)
The value of PI is approximately 3.14159265359.
&gt;&gt;&gt; print &#39;The value of PI is approximately {0!r}.&#39;.format(math.pi)
The value of PI is approximately 3.141592653589793.
</pre></div>
</div>
An optional <tt class="docutils literal">':'</tt> and format specifier can follow the field name. This allows
greater control over how the value is formatted. The following example
truncates Pi to three places after the decimal.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import math
&gt;&gt;&gt; print &#39;The value of PI is approximately {0:.3f}.&#39;.format(math.pi)
The value of PI is approximately 3.142.
</pre></div>
</div>
Passing an integer after the <tt class="docutils literal">':'</tt> will cause that field to be a minimum
number of characters wide. This is useful for making tables pretty.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; table = {&#39;Sjoerd&#39;: 4127, &#39;Jack&#39;: 4098, &#39;Dcab&#39;: 7678}
&gt;&gt;&gt; for name, phone in table.items():
... print &#39;{0:10} ==&gt; {1:10d}&#39;.format(name, phone)
...
Jack ==&gt; 4098
Dcab ==&gt; 7678
Sjoerd ==&gt; 4127
</pre></div>
</div>
If you have a really long format string that you don&#8217;t want to split up, it
would be nice if you could reference the variables to be formatted by name
instead of by position. This can be done by simply passing the dict and using
square brackets <tt class="docutils literal">'[]'</tt> to access the keys
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; table = {&#39;Sjoerd&#39;: 4127, &#39;Jack&#39;: 4098, &#39;Dcab&#39;: 8637678}
&gt;&gt;&gt; print (&#39;Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; &#39;
... &#39;Dcab: {0[Dcab]:d}&#39;.format(table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
</pre></div>
</div>
This could also be done by passing the table as keyword arguments with the &#8216;**&#8217;
notation.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; table = {&#39;Sjoerd&#39;: 4127, &#39;Jack&#39;: 4098, &#39;Dcab&#39;: 8637678}
&gt;&gt;&gt; print &#39;Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}&#39;.format(**table)
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
</pre></div>
</div>
This is particularly useful in combination with the new built-in <a title="vars" class="reference external" href="../library/functions.html#vars"><tt class="xref docutils literal">vars()</tt></a>
function, which returns a dictionary containing all local variables.
For a complete overview of string formatting with <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">str.format()</tt></a>, see
<a class="reference external" href="../library/string.html#formatstrings">Format String Syntax</a>.
<div class="section" id="old-string-formatting">
<h3>7.1.1. Old string formatting<a class="headerlink" href="#old-string-formatting" title="Permalink to this headline">¶</a></h3>
The <tt class="docutils literal">%</tt> operator can also be used for string formatting. It interprets the
left argument much like a <tt class="xref docutils literal">sprintf()</tt>-style format string to be applied
to the right argument, and returns the string resulting from this formatting
operation. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import math
&gt;&gt;&gt; print &#39;The value of PI is approximately %5.3f.&#39; % math.pi
The value of PI is approximately 3.142.
</pre></div>
</div>
Since <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">str.format()</tt></a> is quite new, a lot of Python code still uses the <tt class="docutils literal">%</tt>
operator. However, because this old style of formatting will eventually be
removed from the language, <a title="str.format" class="reference external" href="../library/stdtypes.html#str.format"><tt class="xref docutils literal">str.format()</tt></a> should generally be used.
More information can be found in the <a class="reference external" href="../library/stdtypes.html#string-formatting">String Formatting Operations</a> section.
</div>
</div>
<div class="section" id="reading-and-writing-files">
<h2>7.2. Reading and Writing Files<a class="headerlink" href="#reading-and-writing-files" title="Permalink to this headline">¶</a></h2>
<a title="open" class="reference external" href="../library/functions.html#open"><tt class="xref docutils literal">open()</tt></a> returns a file object, and is most commonly used with two
arguments: <tt class="docutils literal">open(filename, mode)</tt>.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f = open(&#39;/tmp/workfile&#39;, &#39;w&#39;)
&gt;&gt;&gt; print f
&lt;open file &#39;/tmp/workfile&#39;, mode &#39;w&#39; at 80a0960&gt;
</pre></div>
</div>
The first argument is a string containing the filename. The second argument is
another string containing a few characters describing the way in which the file
will be used. mode can be <tt class="docutils literal">'r'</tt> when the file will only be read, <tt class="docutils literal">'w'</tt>
for only writing (an existing file with the same name will be erased), and
<tt class="docutils literal">'a'</tt> opens the file for appending; any data written to the file is
automatically added to the end. <tt class="docutils literal">'r+'</tt> opens the file for both reading and
writing. The mode argument is optional; <tt class="docutils literal">'r'</tt> will be assumed if it&#8217;s
omitted.
On Windows, <tt class="docutils literal">'b'</tt> appended to the mode opens the file in binary mode, so there
are also modes like <tt class="docutils literal">'rb'</tt>, <tt class="docutils literal">'wb'</tt>, and <tt class="docutils literal">'r+b'</tt>. Python on Windows makes
a distinction between text and binary files; the end-of-line characters in text
files are automatically altered slightly when data is read or written. This
behind-the-scenes modification to file data is fine for ASCII text files, but
it&#8217;ll corrupt binary data like that in <tt class="docutils literal">JPEG</tt> or <tt class="docutils literal">EXE</tt> files. Be
very careful to use binary mode when reading and writing such files. On Unix,
it doesn&#8217;t hurt to append a <tt class="docutils literal">'b'</tt> to the mode, so you can use it
platform-independently for all binary files.
<div class="section" id="methods-of-file-objects">
<h3>7.2.1. Methods of File Objects<a class="headerlink" href="#methods-of-file-objects" title="Permalink to this headline">¶</a></h3>
The rest of the examples in this section will assume that a file object called
<tt class="docutils literal">f</tt> has already been created.
To read a file&#8217;s contents, call <tt class="docutils literal">f.read(size)</tt>, which reads some quantity of
data and returns it as a string. size is an optional numeric argument. When
size is omitted or negative, the entire contents of the file will be read and
returned; it&#8217;s your problem if the file is twice as large as your machine&#8217;s
memory. Otherwise, at most size bytes are read and returned. If the end of
the file has been reached, <tt class="docutils literal">f.read()</tt> will return an empty string (<tt class="docutils literal">&quot;&quot;</tt>).
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f.read()
&#39;This is the entire file.\n&#39;
&gt;&gt;&gt; f.read()
&#39;&#39;
</pre></div>
</div>
<tt class="docutils literal">f.readline()</tt> reads a single line from the file; a newline character (<tt class="docutils literal">\n</tt>)
is left at the end of the string, and is only omitted on the last line of the
file if the file doesn&#8217;t end in a newline. This makes the return value
unambiguous; if <tt class="docutils literal">f.readline()</tt> returns an empty string, the end of the file
has been reached, while a blank line is represented by <tt class="docutils literal">'\n'</tt>, a string
containing only a single newline.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f.readline()
&#39;This is the first line of the file.\n&#39;
&gt;&gt;&gt; f.readline()
&#39;Second line of the file\n&#39;
&gt;&gt;&gt; f.readline()
&#39;&#39;
</pre></div>
</div>
<tt class="docutils literal">f.readlines()</tt> returns a list containing all the lines of data in the file.
If given an optional parameter sizehint, it reads that many bytes from the
file and enough more to complete a line, and returns the lines from that. This
is often used to allow efficient reading of a large file by lines, but without
having to load the entire file in memory. Only complete lines will be returned.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f.readlines()
[&#39;This is the first line of the file.\n&#39;, &#39;Second line of the file\n&#39;]
</pre></div>
</div>
An alternative approach to reading lines is to loop over the file object. This is
memory efficient, fast, and leads to simpler code:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for line in f:
 print line,

This is the first line of the file.
Second line of the file
</pre></div>
</div>
The alternative approach is simpler but does not provide as fine-grained
control. Since the two approaches manage line buffering differently, they
should not be mixed.
<tt class="docutils literal">f.write(string)</tt> writes the contents of string to the file, returning
<tt class="xref docutils literal">None</tt>.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f.write(&#39;This is a test\n&#39;)
</pre></div>
</div>
To write something other than a string, it needs to be converted to a string
first:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; value = (&#39;the answer&#39;, 42)
&gt;&gt;&gt; s = str(value)
&gt;&gt;&gt; f.write(s)
</pre></div>
</div>
<tt class="docutils literal">f.tell()</tt> returns an integer giving the file object&#8217;s current position in the
file, measured in bytes from the beginning of the file. To change the file
object&#8217;s position, use <tt class="docutils literal">f.seek(offset, from_what)</tt>. The position is computed
from adding offset to a reference point; the reference point is selected by
the from_what argument. A from_what value of 0 measures from the beginning
of the file, 1 uses the current file position, and 2 uses the end of the file as
the reference point. from_what can be omitted and defaults to 0, using the
beginning of the file as the reference point.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f = open(&#39;/tmp/workfile&#39;, &#39;r+&#39;)
&gt;&gt;&gt; f.write(&#39;0123456789abcdef&#39;)
&gt;&gt;&gt; f.seek(5) # Go to the 6th byte in the file
&gt;&gt;&gt; f.read(1)
&#39;5&#39;
&gt;&gt;&gt; f.seek(-3, 2) # Go to the 3rd byte before the end
&gt;&gt;&gt; f.read(1)
&#39;d&#39;
</pre></div>
</div>
When you&#8217;re done with a file, call <tt class="docutils literal">f.close()</tt> to close it and free up any
system resources taken up by the open file. After calling <tt class="docutils literal">f.close()</tt>,
attempts to use the file object will automatically fail.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; f.close()
&gt;&gt;&gt; f.read()
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ValueError: I/O operation on closed file
</pre></div>
</div>
It is good practice to use the <a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal">with</tt></a> keyword when dealing with file
objects. This has the advantage that the file is properly closed after its
suite finishes, even if an exception is raised on the way. It is also much
shorter than writing equivalent <a class="reference external" href="../reference/compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a>-<a class="reference external" href="../reference/compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> blocks:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; with open(&#39;/tmp/workfile&#39;, &#39;r&#39;) as f:
... read_data = f.read()
&gt;&gt;&gt; f.closed
True
</pre></div>
</div>
File objects have some additional methods, such as <a title="file.isatty" class="reference external" href="../library/stdtypes.html#file.isatty"><tt class="xref docutils literal">isatty()</tt></a> and
<a title="file.truncate" class="reference external" href="../library/stdtypes.html#file.truncate"><tt class="xref docutils literal">truncate()</tt></a> which are less frequently used; consult the Library
Reference for a complete guide to file objects.
</div>
<div class="section" id="the-pickle-module">
<h3>7.2.2. The <a title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref docutils literal">pickle</tt></a> Module<a class="headerlink" href="#the-pickle-module" title="Permalink to this headline">¶</a></h3>
Strings can easily be written to and read from a file. Numbers take a bit more
effort, since the <tt class="xref docutils literal">read()</tt> method only returns strings, which will have to
be passed to a function like <a title="int" class="reference external" href="../library/functions.html#int"><tt class="xref docutils literal">int()</tt></a>, which takes a string like <tt class="docutils literal">'123'</tt>
and returns its numeric value 123. However, when you want to save more complex
data types like lists, dictionaries, or class instances, things get a lot more
complicated.
Rather than have users be constantly writing and debugging code to save
complicated data types, Python provides a standard module called <a title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref docutils literal">pickle</tt></a>.
This is an amazing module that can take almost any Python object (even some
forms of Python code!), and convert it to a string representation; this process
is called pickling. Reconstructing the object from the string
representation is called unpickling. Between pickling and unpickling,
the string representing the object may have been stored in a file or data, or
sent over a network connection to some distant machine.
If you have an object <tt class="docutils literal">x</tt>, and a file object <tt class="docutils literal">f</tt> that&#8217;s been opened for
writing, the simplest way to pickle the object takes only one line of code:
<div class="highlight-python"><div class="highlight"><pre>pickle.dump(x, f)
</pre></div>
</div>
To unpickle the object again, if <tt class="docutils literal">f</tt> is a file object which has been opened
for reading:
<div class="highlight-python"><div class="highlight"><pre>x = pickle.load(f)
</pre></div>
</div>
(There are other variants of this, used when pickling many objects or when you
don&#8217;t want to write the pickled data to a file; consult the complete
documentation for <a title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref docutils literal">pickle</tt></a> in the Python Library Reference.)
<a title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref docutils literal">pickle</tt></a> is the standard way to make Python objects which can be stored and
reused by other programs or by a future invocation of the same program; the
technical term for this is a persistent object. Because <a title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref docutils literal">pickle</tt></a> is
so widely used, many authors who write Python extensions take care to ensure
that new data types such as matrices can be properly pickled and unpickled.
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<li><a class="reference external" href="#">7. Input and Output</a><ul>
<li><a class="reference external" href="#fancier-output-formatting">7.1. Fancier Output Formatting</a><ul>
<li><a class="reference external" href="#old-string-formatting">7.1.1. Old string formatting</a></li>
</ul>
</li>
<li><a class="reference external" href="#reading-and-writing-files">7.2. Reading and Writing Files</a><ul>
<li><a class="reference external" href="#methods-of-file-objects">7.2.1. Methods of File Objects</a></li>
<li><a class="reference external" href="#the-pickle-module">7.2.2. The <tt class="docutils literal">pickle</tt> Module</a></li>
</ul>
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