3. 输入与输出#
Python中格式化输出方法有：
字符串的 str.format() 方法
3.1. 格式化字符串字面值#
格式化字符串字面值 （简称为 f-字符串）在字符串前加前缀 f 或 F，通过 {expression} 表达式，把 Python 表达式的值添加到字符串内。
格式说明符是可选的，写在表达式后面，可以更好地控制格式化值的方式。下例将 pi 舍入到小数点后三位：
import math
print(f'The value of pi is approximately {math.pi:.3f}.')
The value of pi is approximately 3.142.
在 ‘:’ 后传递整数，为该字段设置最小字符宽度，常用于列对齐：
在 ‘:’ 后传递整数，为该字段设置最小字符宽度，常用于列对齐：
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
还有一些修饰符可以在格式化前转换值。 ‘!a’ 应用 ascii() ，’!s’ 应用 str()，’!r’ 应用 repr()：
animals = 'eels'
print(f'My hovercraft is full of {animals}.')
print(f'My hovercraft is full of {animals!r}.')
My hovercraft is full of eels.
My hovercraft is full of 'eels'.
= 说明符可被用于将一个表达式扩展为表达式文本、等号再加表达式求值结果的形式。
形式为 f’{expr=}’ 的 f-字符串将扩展表示为表达式文本，加一个等于号，再加表达式的求值结果
bugs = 'roaches'
count = 13
area = 'living room'
print(f'Debugging {bugs=} {count=} {area=}')
Debugging bugs='roaches' count=13 area='living room'
3.2. 字符串 format() 方法#
花括号及之内的字符（称为格式字段）被替换为传递给 str.format() 方法的对象
花括号中的数字表示传递给 str.format() 方法的对象所在的位置
可以使用关键字参数名引用值
更多使用方法见 格式字符串语法
print('We are the {} who say "{}!"'.format('knights', 'Ni'))
print('{0} and {1}'.format('spam', 'eggs'))
print('{1} and {0}'.format('spam', 'eggs'))
print('This {food} is {adjective}.'.format(
food='spam', adjective='absolutely horrible'))
We are the knights who say "Ni!"
spam and eggs
eggs and spam
This spam is absolutely horrible.
位置参数和关键字参数可以任意组合
print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg'))
The story of Bill, Manfred, and Georg.
可以通过将 table 字典作为采用 ** 标记的关键字参数传入：
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
# 包含所有局部变量的字典
table = {k: str(v) for k, v in vars().items()}
message = " ".join([f'{k}: ' + '{' + k +'};' for k in table.keys()])
print(message.format(**table))
__name__: __main__; __doc__: Automatically created module for IPython interactive environment; __package__: None; __loader__: None; __spec__: None; __builtin__: <module 'builtins' (built-in)>; __builtins__: <module 'builtins' (built-in)>; _ih: ['', "import math\n\nprint(f'The value of pi is approximately {math.pi:.3f}.')", "table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}\n\nfor name, phone in table.items():\n print(f'{name:10} ==> {phone:10d}')", "animals = 'eels'\nprint(f'My hovercraft is full of {animals}.')\nprint(f'My hovercraft is full of {animals!r}.')", "bugs = 'roaches'\ncount = 13\narea = 'living room'\nprint(f'Debugging {bugs=} {count=} {area=}')", 'print(\'We are the {} who say "{}!"\'.format(\'knights\', \'Ni\'))\nprint(\'{0} and {1}\'.format(\'spam\', \'eggs\'))\nprint(\'{1} and {0}\'.format(\'spam\', \'eggs\'))\n\nprint(\'This {food} is {adjective}.\'.format(\n food=\'spam\', adjective=\'absolutely horrible\'))', "print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg'))", "table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}\nprint('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))", '# 包含所有局部变量的字典\ntable = {k: str(v) for k, v in vars().items()}\nmessage = " ".join([f\'{k}: \' + \'{\' + k +\'};\' for k in table.keys()])\nprint(message.format(**table))']; _oh: {}; _dh: [PosixPath('/var/www/the-way-to-python')]; In: ['', "import math\n\nprint(f'The value of pi is approximately {math.pi:.3f}.')", "table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}\n\nfor name, phone in table.items():\n print(f'{name:10} ==> {phone:10d}')", "animals = 'eels'\nprint(f'My hovercraft is full of {animals}.')\nprint(f'My hovercraft is full of {animals!r}.')", "bugs = 'roaches'\ncount = 13\narea = 'living room'\nprint(f'Debugging {bugs=} {count=} {area=}')", 'print(\'We are the {} who say "{}!"\'.format(\'knights\', \'Ni\'))\nprint(\'{0} and {1}\'.format(\'spam\', \'eggs\'))\nprint(\'{1} and {0}\'.format(\'spam\', \'eggs\'))\n\nprint(\'This {food} is {adjective}.\'.format(\n food=\'spam\', adjective=\'absolutely horrible\'))', "print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg'))", "table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}\nprint('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))", '# 包含所有局部变量的字典\ntable = {k: str(v) for k, v in vars().items()}\nmessage = " ".join([f\'{k}: \' + \'{\' + k +\'};\' for k in table.keys()])\nprint(message.format(**table))']; Out: {}; get_ipython: <bound method InteractiveShell.get_ipython of <ipykernel.zmqshell.ZMQInteractiveShell object at 0x7f071e3a50f0>>; exit: <IPython.core.autocall.ZMQExitAutocall object at 0x7f071e3a5a20>; quit: <IPython.core.autocall.ZMQExitAutocall object at 0x7f071e3a5a20>; open: <function open at 0x7f072fbee290>; _: ; __: ; ___: ; _i: table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)); _ii: print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg')); _iii: print('We are the {} who say "{}!"'.format('knights', 'Ni'))
print('{0} and {1}'.format('spam', 'eggs'))
print('{1} and {0}'.format('spam', 'eggs'))
print('This {food} is {adjective}.'.format(
food='spam', adjective='absolutely horrible')); _i1: import math
print(f'The value of pi is approximately {math.pi:.3f}.'); math: <module 'math' (built-in)>; _i2: table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
for name, phone in table.items():
print(f'{name:10} ==> {phone:10d}'); table: {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}; name: Dcab; phone: 7678; _i3: animals = 'eels'
print(f'My hovercraft is full of {animals}.')
print(f'My hovercraft is full of {animals!r}.'); animals: eels; _i4: bugs = 'roaches'
count = 13
area = 'living room'
print(f'Debugging {bugs=} {count=} {area=}'); bugs: roaches; count: 13; area: living room; _i5: print('We are the {} who say "{}!"'.format('knights', 'Ni'))
print('{0} and {1}'.format('spam', 'eggs'))
print('{1} and {0}'.format('spam', 'eggs'))
print('This {food} is {adjective}.'.format(
food='spam', adjective='absolutely horrible')); _i6: print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg')); _i7: table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)); _i8: # 包含所有局部变量的字典
table = {k: str(v) for k, v in vars().items()}
message = " ".join([f'{k}: ' + '{' + k +'};' for k in table.keys()])
print(message.format(**table));
# 打印给定的整数及其平方与立方
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
3.3. str() 与 repr()#
str() 函数返回供人阅读的值，repr() 则生成适于解释器读取的值（如果没有等效的语法，则强制执行 SyntaxError）。对于没有支持供人阅读展示结果的对象， str() 返回与 repr() 相同的值。一般情况下，数字、列表或字典等结构的值，使用这两个函数输出的表现形式是一样的。字符串有两种不同的表现形式。
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...
hello = 'hello, world\n'
hellos = repr(hello)
print(hellos)
'hello, world\n'
repr((x, y, ('spam', 'eggs')))
"(32.5, 40000, ('spam', 'eggs'))"
3.4. 读写文件#
打开文件语法：
open(filename, mode, encoding=None)
filename是文件名字符串
mode是包含描述文件使用方式字符的字符串，省略时的默认值为 ‘r’
‘r’ ，表示文件只能读取
‘w’ 表示只能写入（现有同名文件会被覆盖）
‘a’ 表示打开文件并追加内容，任何写入的数据会自动添加到文件末尾
‘r+’ 表示打开文件进行读写
‘b’ 可以用 binary mode 打开文件，二进制模式的数据是以 bytes 对象的形式读写的
encoding指定待打开文件的编码，如果没有指定 encoding ，默认的是与平台有关的。在二进制模式下打开文件时，你不能指定 encoding 。
在文本模式下读取文件时，默认把平台特定的行结束符（Unix 上为 \n, Windows 上为 \r\n）转换为 \n。在文本模式下写入数据时，默认把 \n 转换回平台特定结束符。这种操作方式在后台修改文件数据对文本文件来说没有问题，但会破坏 JPEG 或 EXE 等二进制文件中的数据。注意，在读写此类文件时，一定要使用二进制模式。
处理文件对象时，最好使用 with 关键字。优点是，子句体结束后，文件会正确关闭，即便触发异常也可以。而且，使用 with 相比等效的 try-finally 代码块要简短得多：
with open('workfile', encoding="utf-8") as f:
read_data = f.read()
# 我们可以检测文件是否已被自动关闭。
f.closed
---------------------------------------------------------------------------
FileNotFoundError Traceback (most recent call last)
Cell In[17], line 1
----> 1 with open('workfile', encoding="utf-8") as f:
2 read_data = f.read()
4 # 我们可以检测文件是否已被自动关闭。
File ~/myenv/lib/python3.10/site-packages/IPython/core/interactiveshell.py:324, in _modified_open(file, *args, **kwargs)
317 if file in {0, 1, 2}:
318 raise ValueError(
319 f"IPython won't let you open fd={file} by default "
320 "as it is likely to crash IPython. If you know what you are doing, "
321 "you can use builtins' open."
322 )
--> 324 return io_open(file, *args, **kwargs)
FileNotFoundError: [Errno 2] No such file or directory: 'workfile'
3.4.1. f.read(size)#
.read(size) 可用于读取文件内容，它会读取一些数据，并返回字符串str（文本模式），或字节串对象bytes（在二进制模式下）。 size 是可选的数值参数。省略 size 或 size 为负数时，读取并返回整个文件的内容。如已到达文件末尾，f.read() 返回空字符串（’’）。
3.4.2. f.readline()#
f.readline() 从文件中读取单行数据；字符串末尾保留换行符（\n），只有在文件不以换行符结尾时，文件的最后一行才会省略换行符。这种方式让返回值清晰明确；只要 f.readline() 返回空字符串，就表示已经到达了文件末尾，空行使用 ‘\n’ 表示，该字符串只包含一个换行符。
从文件中读取多行时，可以用循环遍历整个文件对象。这种操作能高效利用内存，快速，且代码简单：
for line in f:
print(line, end='')