python内置函数之print
Python内置函数 builtins.print 基本用法。
- builtins.repr
- builtins.print
在 Python 2 中，print 为关键字；在 Python 3 中，print 为内置函数（builtins.print）。
builtins 的 print() 函数：
在 Python 3 中可执行 help(print) 查看 print 函数说明：
>>> help(print)
Help on built-in function print in module builtins:
print(...)
print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
Prints the values to a stream, or to sys.stdout by default.
Optional keyword arguments:
file: a file-like object (stream); defaults to the current sys.stdout.
sep: string inserted between values, default a space.
end: string appended after the last value, default a newline.
flush: whether to forcibly flush the stream.
Python3 已经不支持 print 的非函数格式了，必须使用 print() 函数调用格式。
>>> print 'hello,world!'
File "<stdin>", line 1
print 'hello,world!'
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('hello,world!')?
对很多应用程序来说，使用模块 logging 来写入日志比使用 print 更合适。
相关模块参考：syslog, logging。
sep & end#
sep#
print 一行打印多个变量（逗号分隔），默认会以空格（sep=' '）分隔：
可以指定 sep 参数，定制行内分隔（连接）符：
另外一个例子：print 打印日志起始行 tuple，两行之间插了一个空格：
mbr_start_line = '2019-09-22 21:41:59.416 Debug|1031|137920|:96|IMPDT_MBR_Engine||start: role = 1\n'
mbr_stop_line = '2019-09-22 21:43:59.947 Debug|1031|672495|:125|IMPDT_MBR_Engine||stop: reset role from 1\n'
print(mbr_start_line, mbr_stop_line)
2019-09-22 21:41:59.416 Debug|1031|137920|:96|IMPDT_MBR_Engine||start: role = 1
2019-09-22 21:43:59.947 Debug|1031|672495|:125|IMPDT_MBR_Engine||stop: reset role from 1
也可以指定 sep 去掉空格：print(mbr_start_line, mbr_stop_line,sep='')
end#
mbr_start_line = '2019-09-22 21:41:59.416 Debug|1031|137920|:96|IMPDT_MBR_Engine||start: role = 1\n'
mbr_stop_line = '2019-09-22 21:43:59.947 Debug|1031|672495|:125|IMPDT_MBR_Engine||stop: reset role from 1\n'
#print(mbr_start_line),print(mbr_stop_line)
mbr_border_lines = (mbr_start_line, mbr_stop_line) # make tuple
for line in mbr_border_lines:
print(line,end='') #print(line)
打印结果为：
2019-09-22 21:41:59.416 Debug|1031|137920|:96|IMPDT_MBR_Engine||start: role = 1
2019-09-22 21:43:59.947 Debug|1031|672495|:125|IMPDT_MBR_Engine||stop: reset role from 1
由于按文本读出的行末尾本身就有换行符，因此在连续打印多行日志时，可考虑将 print 在尾部追加的换行符置空，或定制其他行间分隔符。
将
print(line)修改为print(line, end='')即可。
expr#
print(var) 或 print(expr) 打印变量或表达式的值。
>>> start=1
>>> stop=10
>>> step=2
>>> print(start)
1
>>> print(start+step)
3
>>> r=range(1,10,2)
# python2
>>> print(r)
[1, 3, 5, 7, 9]
# python3
>>> print(r)
range(1, 10, 2)
>>> l1=[1,2,3,4]
>>> print(l1)
[1, 2, 3, 4]
repr#
print(object) 输出类型或描述信息。
#######################################
# 打印模块信息
#######################################
# python3
>>> print(builtins)
<module 'builtins' (built-in)>
>>> print(array)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'array' is not defined
>>> import array
# python2
>>> print(array)
<module 'array' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/array.so'>
# python3
>>> print(array)
<module 'array' from '/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/lib-dynload/array.cpython-36m-darwin.so'>
除此之外，调用 repr(obj) 可以获取该 obj 对象实例的描述信息。
repr 实际上调用的是 obj 的 __repr__ 方法：repr(obj) = obj.__repr__()
>>> help(repr)
Help on built-in function repr in module builtins:
repr(obj, /)
Return the canonical string representation of the object.
For many object types, including most builtins, eval(repr(obj)) == obj.
部分示例如下：
#######################################
# 打印模块类型信息
#######################################
>>> print(list)
<class 'list'>
>>> repr(list)
"<class 'list'>"
>>> print(len)
<built-in function len>
>>> repr(len)
'<built-in function len>'
>>> print(hex)
<built-in function hex>
>>> repr(hex)
'<built-in function hex>'
#######################################
# 打印对象实例描述（__repr__）
#######################################
>>> print(sys.version_info)
sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)
>>> repr(sys.version_info)
"sys.version_info(major=3, minor=9, micro=13, releaselevel='final', serial=0)"
>>> print(sys.thread_info)
sys.thread_info(name='pthread', lock='mutex+cond', version=None)
>>> repr(sys.thread_info)
"sys.thread_info(name='pthread', lock='mutex+cond', version=None)"
list vars#
逗号后直接接变量，Python 2 下输出 tuple；Python 3 将自动拼接前半句字面量和后面的变量，以空格（space-separated with sep=' '）隔开。
>>> start=0
>>> stop=30
>>> step=5
# python2 下的输出 tuple
>>> print('start is', start)
('start is', 0)
>>> print('start, stop is', start, stop)
('start, stop is', 0, 30)
# python3 下的输出
>>> print('start is', start)
start is 0
>>> print('start, stop is', start, stop)
start, stop is 0 30
变量也可穿插在中间：
>>> name = "John"
>>> score = 85
>>> print("Total score for", name, "is", score)
Total score for John is 85
format#
可参考 builtins.format 和 str.format 函数，输入 help('FORMATTING') 可查看字符串格式化相关议题。
printf-style#
printf-style String Formatting
C printf 风格，使用 % 格式符标识占位，
# start 可加括号
>>> print('start is %d' % start)
start is 0
>>> print('start,stop is %d, %d' % (start, stop))
start,stop is 0, 30
以下采用 C-printf-style，控制字符串截断输出：
>>> 'i love %s' % 'python'
'i love python'
>>> 'i love %.2s' % 'python'
'i love py'
# 截取两位，总共三位，默认右对齐
>>> 'i love %3.2s' % 'python'
'i love py'
# 截取两位，总共十位，默认右对齐
>>> 'i love %10.2s' % 'python'
'i love py'
# 截取两位，总共十位，设置左对齐
>>> 'i love %-10.2s' % 'python'
'i love py '
# 截取两位，总共十位，设置左对齐
>>> 'i love %-10.2s !' % 'python'
'i love py !'
format specifier#
在占位序号或关键字之后可以 : 来设置填充（fill）、对齐（align）、位宽（width）、进制（base）等格式控制。
An optional '
:' and format specifier can follow the field name. This allows greater control over how the value is formatted.
- 设置打印进制格式(str.format)
>>> # format also supports binary numbers
>>> "int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}".format(42)
'int: 42; hex: 2a; oct: 52; bin: 101010'
>>> # with 0x, 0o, or 0b as prefix:
>>> "int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}".format(42)
'int: 42; hex: 0x2a; oct: 0o52; bin: 0b101010'
- 设置浮点数位数(str.format)
>>> import math
>>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
The value of PI is approximately 3.142.
>>> '{0:6.2f}'.format(math.pi)
' 3.14'
>>> '{0:>6.2f}'.format(math.pi)
' 3.14'
>>> '{0:<6.2f}'.format(math.pi)
'3.14 '
-
设置输出宽度及填充格式(str.format)
Aligning the text and specifying a width
# default align left, fill right with spaces
>>> '{:30}'.format('left aligned')
'left aligned '
>>> '{:<30}'.format('left aligned')
'left aligned '
>>> '{:>30}'.format('right aligned')
' right aligned'
>>> '{:^30}'.format('centered')
' centered '
>>> '{:*^30}'.format('centered') # use '*' as a fill char
'***********centered***********'
>>> '{:-^30}'.format('centered') # use '-' as a fill char
'-----------centered-----------'
Passing an integer after the '
:' will cause that field to be a minimum number of characters wide. This is useful for making tables pretty.
# position:{width}{base}
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print('{0:10} ==> {1:10d}'.format(name, phone))
...
Sjoerd ==> 4127
Jack ==> 4098
Dcab ==> 7678
- 逗号千位分隔（Using the comma as a thousands separator）
# 对整数进行千位分隔
>>> str1 = '{:,}'.format(1234567890)
>>> str1
'1,234,567,890'
# 去掉千位分隔符
>>> int(str1.replace(',', ''))
1234567890
- 输出百分数(str.format)
>>> points = 19
>>> total = 22
>>> 'Correct rate of answers: {:.2%}'.format(points/total)
'Correct rate of answers: 86.36%'