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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>7. 輸入和輸出 — Python 3.7.0 說明文件</title>

title="在 Python 3.7.0 說明文件中搜尋"

href="../_static/opensearch.xml"/>

</head><body>

<ul>

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accesskey="I">索引</a></li>

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</div>

<h1>7. 輸入和輸出<a class="headerlink" href="#input-and-output" title="本標題的永久連結">¶</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.

<h2>7.1. Fancier Output Formatting<a class="headerlink" href="#fancier-output-formatting" title="本標題的永久連結">¶</a></h2>

So far we’ve encountered two ways of writing values: expression statements and

the <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> function. (A third way is using the <code class="xref py py-meth docutils literal notranslate">write()</code> method

of file objects; the standard output file can be referenced as <code class="docutils literal notranslate">sys.stdout</code>.

See the Library Reference for more information on this.)

Often you’ll want more control over the formatting of your output than simply

printing space-separated values. There are several ways to format output.

<ul>

<li>To use <a class="reference internal" href="#tut-f-strings">formatted string literals</a>, begin a string

with <code class="docutils literal notranslate">f</code> or <code class="docutils literal notranslate">F</code> before the opening quotation mark or triple quotation mark.

Inside this string, you can write a Python expression between <code class="docutils literal notranslate">{</code> and <code class="docutils literal notranslate">}</code>

characters that can refer to variables or literal values.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> year = 2016 ; event = 'Referendum'

>>> f'Results of the {year} {event}'

'Results of the 2016 Referendum'

</pre></div>

</div>

</li>

<li>The <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method of strings requires more manual

effort. You’ll still use <code class="docutils literal notranslate">{</code> and <code class="docutils literal notranslate">}</code> to mark where a variable

will be substituted and can provide detailed formatting directives,

but you’ll also need to provide the information to be formatted.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> yes_votes = 42_572_654 ; no_votes = 43_132_495

>>> percentage = yes_votes/(yes_votes+no_votes)

>>> '{:-9} YES votes {:2.2%}'.format(yes_votes, percentage)

' 42572654 YES votes 49.67%'

</pre></div>

</div>

</li>

<li>Finally, you can do all the string handling yourself by using string slicing and

concatenation operations to create any layout you can imagine. The

string type has some methods that perform useful operations for padding

strings to a given column width.

</li>

</ul>

When you don’t need fancy output but just want a quick display of some

variables for debugging purposes, you can convert any value to a string with

the <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> or <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a> functions.

The <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a> function is meant to return representations of values which are

fairly human-readable, while <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> is meant to generate representations

which can be read by the interpreter (or will force a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> if

there is no equivalent syntax). For objects which don’t have a particular

representation for human consumption, <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a> will return the same value as

<a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a>. Many values, such as numbers or structures like lists and

dictionaries, have the same representation using either function. Strings, in

particular, have two distinct representations.

一些範例：

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> s = 'Hello, world.'

>>> str(s)

'Hello, world.'

>>> repr(s)

"'Hello, world.'"

>>> str(1/7)

'0.14285714285714285'

>>> x = 10 * 3.25

>>> y = 200 * 200

>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'

>>> print(s)

The value of x is 32.5, and y is 40000...

>>> # The repr() of a string adds string quotes and backslashes:

... hello = 'hello, world\n'

>>> hellos = repr(hello)

>>> print(hellos)

'hello, world\n'

>>> # The argument to repr() may be any Python object:

... repr((x, y, ('spam', 'eggs')))

"(32.5, 40000, ('spam', 'eggs'))"

</pre></div>

</div>

The <a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal notranslate">string</code></a> module contains a <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal notranslate">Template</code></a> class that offers

yet another way to substitute values into strings, using placeholders like

<code class="docutils literal notranslate">$x</code> and replacing them with values from a dictionary, but offers much less

control of the formatting.

<h3>7.1.1. Formatted String Literals<a class="headerlink" href="#formatted-string-literals" title="本標題的永久連結">¶</a></h3>

<a class="reference internal" href="../reference/lexical_analysis.html#f-strings">Formatted string literals</a> (also called f-strings for

short) let you include the value of Python expressions inside a string by

prefixing the string with <code class="docutils literal notranslate">f</code> or <code class="docutils literal notranslate">F</code> and writing expressions as

<code class="docutils literal notranslate">{expression}</code>.

An optional format specifier can follow the expression. This allows greater

control over how the value is formatted. The following example rounds pi to

three places after the decimal:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import math

>>> print(f'The value of pi is approximately {math.pi:.3f}.')

</pre></div>

</div>

Passing an integer after the <code class="docutils literal notranslate">':'</code> will cause that field to be a minimum

number of characters wide. This is useful for making columns line up.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}

>>> for name, phone in table.items():

... print(f'{name:10} ==> {phone:10d}')

...

Sjoerd ==> 4127

Jack ==> 4098

Dcab ==> 7678

</pre></div>

</div>

Other modifiers can be used to convert the value before it is formatted.

<code class="docutils literal notranslate">'!a'</code> applies <a class="reference internal" href="../library/functions.html#ascii" title="ascii"><code class="xref py py-func docutils literal notranslate">ascii()</code></a>, <code class="docutils literal notranslate">'!s'</code> applies <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a>, and <code class="docutils literal notranslate">'!r'</code>

applies <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> animals = 'eels'

>>> print(f'My hovercraft is full of {animals}.')

My hovercraft is full of eels.

>>> print('My hovercraft is full of {animals !r}.')

My hovercraft is full of 'eels'.

</pre></div>

</div>

For a reference on these format specifications, see

the reference guide for the <a class="reference internal" href="../library/string.html#formatspec">Format Specification Mini-Language</a>.

</div>

<h3>7.1.2. The String format() Method<a class="headerlink" href="#the-string-format-method" title="本標題的永久連結">¶</a></h3>

Basic usage of the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method looks like this:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))

We are the knights who say "Ni!"

</pre></div>

</div>

The brackets and characters within them (called format fields) are replaced with

the objects passed into the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method. A number in the

brackets can be used to refer to the position of the object passed into the

<a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> print('{0} and {1}'.format('spam', 'eggs'))

spam and eggs

>>> print('{1} and {0}'.format('spam', 'eggs'))

eggs and spam

</pre></div>

</div>

If keyword arguments are used in the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method, their values

are referred to by using the name of the argument.

... food='spam', adjective='absolutely horrible'))

This spam is absolutely horrible.

</pre></div>

</div>

Positional and keyword arguments can be arbitrarily combined:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',

other='Georg'))

The story of Bill, Manfred, and Georg.

</pre></div>

</div>

If you have a really long format string that you don’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 <code class="docutils literal notranslate">'[]'</code> to access the keys

>>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '

... 'Dcab: {0[Dcab]:d}'.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 『**』

notation.

>>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))

Jack: 4098; Sjoerd: 4127; Dcab: 8637678

</pre></div>

</div>

This is particularly useful in combination with the built-in function

<a class="reference internal" href="../library/functions.html#vars" title="vars"><code class="xref py py-func docutils literal notranslate">vars()</code></a>, which returns a dictionary containing all local variables.

As an example, the following lines produce a tidily-aligned

set of columns giving integers and their squares and cubes:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> for x in range(1, 11):

... print('{0:2d} {1:3d} {2:4d}'.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>

For a complete overview of string formatting with <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a>, see

<a class="reference internal" href="../library/string.html#formatstrings">Format String Syntax</a>.

</div>

<h3>7.1.3. Manual String Formatting<a class="headerlink" href="#manual-string-formatting" title="本標題的永久連結">¶</a></h3>

Here’s the same table of squares and cubes, formatted manually:

... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')

... # Note use of 'end' on previous line

...

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 the one space between each column was added by the

way <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> works: it always adds spaces between its arguments.)

The <a class="reference internal" href="../library/stdtypes.html#str.rjust" title="str.rjust"><code class="xref py py-meth docutils literal notranslate">str.rjust()</code></a> method of string objects right-justifies a string in a

field of a given width by padding it with spaces on the left. There are

similar methods <a class="reference internal" href="../library/stdtypes.html#str.ljust" title="str.ljust"><code class="xref py py-meth docutils literal notranslate">str.ljust()</code></a> and <a class="reference internal" href="../library/stdtypes.html#str.center" title="str.center"><code class="xref py py-meth docutils literal notranslate">str.center()</code></a>. These methods do

not write anything, they just return a new string. If the input string is too

long, they don’t truncate it, but return it unchanged; this will mess up your

column lay-out but that’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 <code class="docutils literal notranslate">x.ljust(n)[:n]</code>.)

There is another method, <a class="reference internal" href="../library/stdtypes.html#str.zfill" title="str.zfill"><code class="xref py py-meth docutils literal notranslate">str.zfill()</code></a>, which pads a numeric string on the

left with zeros. It understands about plus and minus signs:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> '12'.zfill(5)

'00012'

>>> '-3.14'.zfill(7)

'-003.14'

>>> '3.14159265359'.zfill(5)

'3.14159265359'

</pre></div>

</div>

<h3>7.1.4. Old string formatting<a class="headerlink" href="#old-string-formatting" title="本標題的永久連結">¶</a></h3>

The <code class="docutils literal notranslate">%</code> operator can also be used for string formatting. It interprets the

left argument much like a <code class="xref c c-func docutils literal notranslate">sprintf()</code>-style format string to be applied

to the right argument, and returns the string resulting from this formatting

operation. For example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import math

>>> print('The value of pi is approximately %5.3f.' % math.pi)

The value of pi is approximately 3.142.

</pre></div>

</div>

More information can be found in the <a class="reference internal" href="../library/stdtypes.html#old-string-formatting">printf-style String Formatting</a> section.

</div>

<h2>7.2. Reading and Writing Files<a class="headerlink" href="#reading-and-writing-files" title="本標題的永久連結">¶</a></h2>

<a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a> returns a <a class="reference internal" href="../glossary.html#term-file-object">file object</a>, and is most commonly used with

two arguments: <code class="docutils literal notranslate">open(filename, mode)</code>.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> f = open('workfile', 'w')

</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 <code class="docutils literal notranslate">'r'</code> when the file will only be read, <code class="docutils literal notranslate">'w'</code>

for only writing (an existing file with the same name will be erased), and

<code class="docutils literal notranslate">'a'</code> opens the file for appending; any data written to the file is

automatically added to the end. <code class="docutils literal notranslate">'r+'</code> opens the file for both reading and

writing. The mode argument is optional; <code class="docutils literal notranslate">'r'</code> will be assumed if it’s

omitted.

Normally, files are opened in text mode, that means, you read and write

strings from and to the file, which are encoded in a specific encoding. If

encoding is not specified, the default is platform dependent (see

<a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a>). <code class="docutils literal notranslate">'b'</code> appended to the mode opens the file in

binary mode: now the data is read and written in the form of bytes

objects. This mode should be used for all files that don’t contain text.

In text mode, the default when reading is to convert platform-specific line

endings (<code class="docutils literal notranslate">\n</code> on Unix, <code class="docutils literal notranslate">\r\n</code> on Windows) to just <code class="docutils literal notranslate">\n</code>. When writing in

text mode, the default is to convert occurrences of <code class="docutils literal notranslate">\n</code> back to

platform-specific line endings. This behind-the-scenes modification

to file data is fine for text files, but will corrupt binary data like that in

<code class="file docutils literal notranslate">JPEG</code> or <code class="file docutils literal notranslate">EXE</code> files. Be very careful to use binary mode when

reading and writing such files.

It is good practice to use the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> keyword when dealing

with file objects. The advantage is that the file is properly closed

after its suite finishes, even if an exception is raised at some

point. Using <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> is also much shorter than writing

equivalent <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>-<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> blocks:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> with open('workfile') as f:

... read_data = f.read()

>>> f.closed

True

</pre></div>

</div>

If you’re not using the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> keyword, then you should call

<code class="docutils literal notranslate">f.close()</code> to close the file and immediately free up any system

resources used by it. If you don’t explicitly close a file, Python’s

garbage collector will eventually destroy the object and close the

open file for you, but the file may stay open for a while. Another

risk is that different Python implementations will do this clean-up at

different times.

After a file object is closed, either by a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement

or by calling <code class="docutils literal notranslate">f.close()</code>, attempts to use the file object will

automatically fail.

>>> f.read()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ValueError: I/O operation on closed file.

</pre></div>

</div>

<h3>7.2.1. Methods of File Objects<a class="headerlink" href="#methods-of-file-objects" title="本標題的永久連結">¶</a></h3>

The rest of the examples in this section will assume that a file object called

<code class="docutils literal notranslate">f</code> has already been created.

To read a file’s contents, call <code class="docutils literal notranslate">f.read(size)</code>, which reads some quantity of

data and returns it as a string (in text mode) or bytes object (in binary mode).

size is an optional numeric argument. When size is omitted or negative, the

entire contents of the file will be read and returned; it’s your problem if the

file is twice as large as your machine’s memory. Otherwise, at most size bytes

are read and returned.

If the end of the file has been reached, <code class="docutils literal notranslate">f.read()</code> will return an empty

string (<code class="docutils literal notranslate">''</code>).

'This is the entire file.\n'

>>> f.read()

''

</pre></div>

</div>

<code class="docutils literal notranslate">f.readline()</code> reads a single line from the file; a newline character (<code class="docutils literal notranslate">\n</code>)

is left at the end of the string, and is only omitted on the last line of the

file if the file doesn’t end in a newline. This makes the return value

unambiguous; if <code class="docutils literal notranslate">f.readline()</code> returns an empty string, the end of the file

has been reached, while a blank line is represented by <code class="docutils literal notranslate">'\n'</code>, a string

containing only a single newline.

'This is the first line of the file.\n'

>>> f.readline()

'Second line of the file\n'

>>> f.readline()

''

</pre></div>

</div>

For reading lines from a file, you can loop over the file object. This is memory

efficient, fast, and leads to simple code:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> for line in f:

... print(line, end='')

...

This is the first line of the file.

Second line of the file

</pre></div>

</div>

If you want to read all the lines of a file in a list you can also use

<code class="docutils literal notranslate">list(f)</code> or <code class="docutils literal notranslate">f.readlines()</code>.

<code class="docutils literal notranslate">f.write(string)</code> writes the contents of string to the file, returning

the number of characters written.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> f.write('This is a test\n')

15

</pre></div>

</div>

Other types of objects need to be converted – either to a string (in text mode)

or a bytes object (in binary mode) – before writing them:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> value = ('the answer', 42)

>>> s = str(value) # convert the tuple to string

>>> f.write(s)

18

</pre></div>

</div>

<code class="docutils literal notranslate">f.tell()</code> returns an integer giving the file object’s current position in the file

represented as number of bytes from the beginning of the file when in binary mode and

an opaque number when in text mode.

To change the file object’s position, use <code class="docutils literal notranslate">f.seek(offset, from_what)</code>. 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.

>>> f.write(b'0123456789abcdef')

16

>>> f.seek(5) # Go to the 6th byte in the file

5

>>> f.read(1)

b'5'

>>> f.seek(-3, 2) # Go to the 3rd byte before the end

13

>>> f.read(1)

b'd'

</pre></div>

</div>

In text files (those opened without a <code class="docutils literal notranslate">b</code> in the mode string), only seeks

relative to the beginning of the file are allowed (the exception being seeking

to the very file end with <code class="docutils literal notranslate">seek(0, 2)</code>) and the only valid offset values are

those returned from the <code class="docutils literal notranslate">f.tell()</code>, or zero. Any other offset value produces

undefined behaviour.

File objects have some additional methods, such as <code class="xref py py-meth docutils literal notranslate">isatty()</code> and

<code class="xref py py-meth docutils literal notranslate">truncate()</code> which are less frequently used; consult the Library

Reference for a complete guide to file objects.

</div>

<h3>7.2.2. Saving structured data with <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate">json</code></a><a class="headerlink" href="#saving-structured-data-with-json" title="本標題的永久連結">¶</a></h3>

Strings can easily be written to and read from a file. Numbers take a bit more

effort, since the <code class="xref py py-meth docutils literal notranslate">read()</code> method only returns strings, which will have to

be passed to a function like <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a>, which takes a string like <code class="docutils literal notranslate">'123'</code>

and returns its numeric value 123. When you want to save more complex data

types like nested lists and dictionaries, parsing and serializing by hand

becomes complicated.

Rather than having users constantly writing and debugging code to save

complicated data types to files, Python allows you to use the popular data

interchange format called <a class="reference external" href="http://json.org">JSON (JavaScript Object Notation)</a>. The standard module called <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate">json</code></a> can take Python

data hierarchies, and convert them to string representations; this process is

called serializing. Reconstructing the data from the string representation

is called deserializing. Between serializing and deserializing, the

string representing the object may have been stored in a file or data, or

sent over a network connection to some distant machine.

備註

The JSON format is commonly used by modern applications to allow for data

exchange. Many programmers are already familiar with it, which makes

it a good choice for interoperability.

</div>

If you have an object <code class="docutils literal notranslate">x</code>, you can view its JSON string representation with a

simple line of code:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> import json

>>> json.dumps([1, 'simple', 'list'])

'[1, "simple", "list"]'

</pre></div>

</div>

Another variant of the <a class="reference internal" href="../library/json.html#json.dumps" title="json.dumps"><code class="xref py py-func docutils literal notranslate">dumps()</code></a> function, called <a class="reference internal" href="../library/json.html#json.dump" title="json.dump"><code class="xref py py-func docutils literal notranslate">dump()</code></a>,

simply serializes the object to a <a class="reference internal" href="../glossary.html#term-text-file">text file</a>. So if <code class="docutils literal notranslate">f</code> is a

<a class="reference internal" href="../glossary.html#term-text-file">text file</a> object opened for writing, we can do this:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>json.dump(x, f)

</pre></div>

</div>

To decode the object again, if <code class="docutils literal notranslate">f</code> is a <a class="reference internal" href="../glossary.html#term-text-file">text file</a> object which has

been opened for reading:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>x = json.load(f)

</pre></div>

</div>

This simple serialization technique can handle lists and dictionaries, but

serializing arbitrary class instances in JSON requires a bit of extra effort.

The reference for the <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate">json</code></a> module contains an explanation of this.

也參考

<a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> - the pickle module

Contrary to <a class="reference internal" href="#tut-json">JSON</a>, pickle is a protocol which allows

the serialization of arbitrarily complex Python objects. As such, it is

specific to Python and cannot be used to communicate with applications

written in other languages. It is also insecure by default:

deserializing pickle data coming from an untrusted source can execute

arbitrary code, if the data was crafted by a skilled attacker.

</div>

<ul>

<li><a class="reference internal" href="#">7. 輸入和輸出</a><ul>

<li><a class="reference internal" href="#fancier-output-formatting">7.1. Fancier Output Formatting</a><ul>

<li><a class="reference internal" href="#formatted-string-literals">7.1.1. Formatted String Literals</a></li>

<li><a class="reference internal" href="#the-string-format-method">7.1.2. The String format() Method</a></li>

<li><a class="reference internal" href="#manual-string-formatting">7.1.3. Manual String Formatting</a></li>

<li><a class="reference internal" href="#old-string-formatting">7.1.4. Old string formatting</a></li>

</ul>

</li>

<li><a class="reference internal" href="#reading-and-writing-files">7.2. Reading and Writing Files</a><ul>

<li><a class="reference internal" href="#methods-of-file-objects">7.2.1. Methods of File Objects</a></li>

<li><a class="reference internal" href="#saving-structured-data-with-json">7.2.2. Saving structured data with <code class="docutils literal notranslate">json</code></a></li>

</ul>

</li>

</ul>

</li>

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