有几种办法呈现程序输出；数据可以按人类可读形式打印，或写入文件以供未来使用。本章节将讨论一些可能性。
So far we’ve encountered two ways of writing values:
expression statements
和
print()
function. (A third way is using the
write()
method of file objects; the standard output file can be referenced as
sys.stdout
. 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.
要使用
格式化字符串文字
，开始字符串按
f
or
F
before the opening quotation mark or triple quotation mark. Inside this string, you can write a Python expression between
{
and
}
characters that can refer to variables or literal values.
>>> year = 2016 >>> event = 'Referendum' >>> f'Results of the {year} {event}' 'Results of the 2016 Referendum'
str.format()
method of strings requires more manual effort. You’ll still use
{
and
}
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.
>>> 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%'
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.
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
repr()
or
str()
函数。
str()
function is meant to return representations of values which are fairly human-readable, while
repr()
is meant to generate representations which can be read by the interpreter (or will force a
SyntaxError
if there is no equivalent syntax). For objects which don’t have a particular representation for human consumption,
str()
will return the same value as
repr()
. 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.
一些范例：
>>> 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'))"
string
module contains a
Template
class that offers yet another way to substitute values into strings, using placeholders like
$x
and replacing them with values from a dictionary, but offers much less control of the formatting.
格式化字符串文字
(also called f-strings for short) let you include the value of Python expressions inside a string by prefixing the string with
f
or
F
and writing expressions as
{expression}
.
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:
>>> import math >>> print(f'The value of pi is approximately {math.pi:.3f}.') The value of pi is approximately 3.142.
Passing an integer after the
':'
will cause that field to be a minimum number of characters wide. This is useful for making columns line up.
>>> 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
Other modifiers can be used to convert the value before it is formatted.
'!a'
applies
ascii()
,
'!s'
applies
str()
，和
'!r'
applies
repr()
:
>>> animals = 'eels' >>> print(f'My hovercraft is full of {animals}.') My hovercraft is full of eels. >>> print(f'My hovercraft is full of {animals!r}.') My hovercraft is full of 'eels'.
=
specifier can be used to expand an expression to the text of the expression, an equal sign, then the representation of the evaluated expression:
>>> bugs = 'roaches' >>> count = 13 >>> area = 'living room' >>> print(f'Debugging {bugs=} {count=} {area=}') Debugging bugs='roaches' count=13 area='living room'
见
self-documenting expressions
for more information on the
=
specifier. For a reference on these format specifications, see the reference guide for the
格式规范迷你语言
.
基本用法的
str.format()
方法看起来像这样:
>>> print('We are the {} who say "{}!"'.format('knights', 'Ni')) We are the knights who say "Ni!"
The brackets and characters within them (called format fields) are replaced with the objects passed into the
str.format()
method. A number in the brackets can be used to refer to the position of the object passed into the
str.format()
方法。
>>> print('{0} and {1}'.format('spam', 'eggs')) spam and eggs >>> print('{1} and {0}'.format('spam', 'eggs')) eggs and spam
If keyword arguments are used in the
str.format()
method, their values are referred to by using the name of the argument.
>>> print('This {food} is {adjective}.'.format( ... food='spam', adjective='absolutely horrible')) This spam is absolutely horrible.
Positional and keyword arguments can be arbitrarily combined:
>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', ... other='Georg')) The story of Bill, Manfred, and Georg.
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
'[]'
to access the keys.
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} >>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; ' ... 'Dcab: {0[Dcab]:d}'.format(table)) Jack: 4098; Sjoerd: 4127; Dcab: 8637678
This could also be done by passing the
table
dictionary as keyword arguments with the
**
notation.
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} >>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)) Jack: 4098; Sjoerd: 4127; Dcab: 8637678
This is particularly useful in combination with the built-in function
vars()
, 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:
>>> 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
For a complete overview of string formatting with
str.format()
，见
格式字符串语法
.
Here’s the same table of squares and cubes, formatted manually:
>>> for x in range(1, 11): ... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ') ... # Note use of 'end' 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
(Note that the one space between each column was added by the way
print()
works: it always adds spaces between its arguments.)
str.rjust()
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
str.ljust()
and
str.center()
. 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
x.ljust(n)[:n]
)。
There is another method,
str.zfill()
, which pads a numeric string on the left with zeros. It understands about plus and minus signs:
>>> '12'.zfill(5) '00012' >>> '-3.14'.zfill(7) '-003.14' >>> '3.14159265359'.zfill(5) '3.14159265359'
The % operator (modulo) can also be used for string formatting. Given
'string'
% values
, instances of
%
in
string
are replaced with zero or more elements of
values
. This operation is commonly known as string interpolation. For example:
>>> import math >>> print('The value of pi is approximately %5.3f.' % math.pi) The value of pi is approximately 3.142.
可以找到更多信息在 printf 样式字符串格式化 章节。
open()
返回
文件对象
，且最常见的是使用 2 位置自变量和 1 关键词自变量：
open(filename, mode, encoding=None)
>>> f = open('workfile', 'w', encoding="utf-8")
第 1 自变量是包含文件名的字符串。第 2 自变量是包含描述文件使用方式的一些字符的另一字符串。
mode
可以是
'r'
当只读文件时，
'w'
为只写 (将擦除具有相同名称的现有文件)，和
'a'
打开文件以供追加；将要写入文件的任何数据自动添加到末尾。
'r+'
打开文件以供读取和写入两者。
mode
自变量是可选的；
'r'
会被假定若省略。
通常，打开文件是按
文本模式
，意味着从文件读取字符串和将字符串写入文件，编码是按特定
encoding
。若
encoding
未指定，默认从属平台 (见
open()
)。因为 UTF-8 是现代事实标准，
encoding="utf-8"
是推荐的除非知道需要使用不同编码。追加
'b'
到模式打开文件以
二进制模式
。二进制模式数据的读取和写入是按
bytes
对象。不可以指定
encoding
当以二进制模式打开文件时。
当以文本模式读取时，默认转换特定平台行尾 (
\n
在 Unix，
\r\n
在 Windows) 到仅仅
\n
。当以文本模式写入时，默认转换出现的
\n
回特定平台行尾。这种对文件数据的幕后修改适用于文本文件，但会破坏二进制数据像在
JPEG
or
EXE
文件。要非常小心使用二进制模式，当读写这种文件时。
是很好实践使用
with
关键词当处理文件对象时。优点是文件在套件完成后会正确关闭，即使某些时候有引发异常。使用
with
也短得多比编写等效
try
-
finally
块：
>>> with open('workfile', encoding="utf-8") as f: ... read_data = f.read() >>> # We can check that the file has been automatically closed. >>> f.closed True
若不使用
with
关键词，那么应调用
f.close()
来关闭文件并立即释放由它使用的任何系统资源。
警告
调用
f.write()
不使用
with
关键词或调用
f.close()
might
导致自变量对于
f.write()
未被完全写入磁盘，即使程序成功退出。
在文件对象关闭之后，通过
with
语句或通过调用
f.close()
，试图使用文件对象会自动失败。
>>> f.close() >>> f.read() Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: I/O operation on closed file.
The rest of the examples in this section will assume that a file object called
f
has already been created.
To read a file’s contents, call
f.read(size)
, 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
characters (in text mode) or
size
bytes (in binary mode) are read and returned. If the end of the file has been reached,
f.read()
will return an empty string (
''
).
>>> f.read() 'This is the entire file.\n' >>> f.read() ''
f.readline()
reads a single line from the file; a newline character (
\n
) 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
f.readline()
returns an empty string, the end of the file has been reached, while a blank line is represented by
'\n'
, a string containing only a single newline.
>>> f.readline() 'This is the first line of the file.\n' >>> f.readline() 'Second line of the file\n' >>> f.readline() ''
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
>>> for line in f: ... print(line, end='') ... This is the first line of the file. Second line of the file
If you want to read all the lines of a file in a list you can also use
list(f)
or
f.readlines()
.
f.write(string)
writes the contents of
string
to the file, returning the number of characters written.
>>> f.write('This is a test\n') 15
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:
>>> value = ('the answer', 42) >>> s = str(value) # convert the tuple to string >>> f.write(s) 18
f.tell()
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
f.seek(offset, whence)
. The position is computed from adding
offset
to a reference point; the reference point is selected by the
whence
argument. A
whence
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.
whence
can be omitted and defaults to 0, using the beginning of the file as the reference point.
>>> f = open('workfile', 'rb+') >>> 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'
In text files (those opened without a
b
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
seek(0, 2)
) and the only valid
offset
values are those returned from the
f.tell()
, or zero. Any other
offset
value produces undefined behaviour.
File objects have some additional methods, such as
isatty()
and
truncate()
which are less frequently used; consult the Library Reference for a complete guide to file objects.
json
¶
Strings can easily be written to and read from a file. Numbers take a bit more effort, since the
read()
method only returns strings, which will have to be passed to a function like
int()
, which takes a string like
'123'
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
JSON (JavaScript 对象表示法)
. The standard module called
json
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.
注意
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.
If you have an object
x
, you can view its JSON string representation with a simple line of code:
>>> import json >>> x = [1, 'simple', 'list'] >>> json.dumps(x) '[1, "simple", "list"]'
另一变体的
dumps()
函数，称为
dump()
，将对象简单序列化成
文本文件
。因此若
f
是
文本文件
object opened for writing, we can do this:
json.dump(x, f)
To decode the object again, if
f
是
二进制文件
or
文本文件
object which has been opened for reading:
x = json.load(f)
注意
JSON files must be encoded in UTF-8. Use
encoding="utf-8"
when opening JSON file as a
文本文件
for both of reading and writing.
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
json
module contains an explanation of this.
另请参阅
pickle
- 腌制模块
Contrary to JSON , 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.