Python format string

更多請參考： PEP-3101



基本的  format string 使用方法：

# basic
In [1]: "Hello, {}".format("xatier")
Out[1]: 'Hello, xatier'


# {0} means the first argument in format()
In [2]: "Hello, {0}".format("xatier")
Out[2]: 'Hello, xatier'


# {1} means the second argument
In [3]: "{1} {0}".format("xatier", 3)
Out[3]: '3 xatier'


# pass arguments one by one
In [4]: "{} {}".format("xatier", 3)
Out[4]: 'xatier 3'


# use doubled { to represent '{'
In [5]: "{} {{".format("xatier")
Out[5]: 'xatier {'


# get item: use [] to get fields in a dictionary
In [10]: my_dog = {'name': "Jack", 'age': 3}

In [11]: "My dog's name: {0[name]}, {0[age]} years old.".format(my_dog)
Out[11]: "My dog's name: Jack, 3 years old."


# do the same thing in an anonymous dictionary
In [15]: "My name is {0[name]}".format({'name': 'Fred'})
Out[15]: 'My name is Fred'

In [16]: "My name is {0[name]}".format(dict(name='Fred'))
Out[16]: 'My name is Fred'


# get attribute: use '.' to get attributes in a class/object
In [18]: import sys

In [19]: "{0.stdin}".format(sys)
Out[19]: "<open file '<stdin>', mode 'r' at 0x7f88ecd94150>"

In [20]: "{0.stdin.write}".format(sys)
Out[20]: '<built-in method write of file object at 0x7f88ecd94150>'


# get object's data
In [30]: class Foo:
   ....:     def __init__(self, name="yoo"):
   ....:         self.name = name
   ....:

In [31]: a = Foo()

In [32]: "{0.name}".format(a)
Out[32]: 'yoo'

In [33]: a = Foo("my name ^.<")

In [34]: "{0.name}".format(a)
Out[34]: 'my name ^.<'

Only two operators are supported: the '.' (getattr) operator, and the '[]' (getitem) operator.
The reason for allowing these operators is that they don't normally have side effects in non-pathological code.

排版：


The general form of a standard format specifier is:
        [[fill]align][sign][#][0][minimumwidth][.precision][type]

    The brackets ([]) indicate an optional element.

# fill with space
In [42]: "My name is {0:8}".format('Fred')
Out[42]: 'My name is Fred    '


# string force to left/right
In [44]: "My name is {0:<8}".format('Fred')
Out[44]: 'My name is Fred    '

In [45]: "My name is {0:>8}".format('Fred')
Out[45]: 'My name is     Fred'


# floating point
In [72]: "{0:f}".format(123.45)
Out[72]: '123.450000'

In [73]: "{0:.3f}".format(123.45)
Out[73]: '123.450'

In [75]: "{0:>10.3f}".format(123.45)
Out[75]: '   123.450'


# show the + sign
In [76]: "{0:>+10.3f}".format(123.45)
Out[76]: '  +123.450'

In [77]: "{0:<+10.3f}".format(123.45)
Out[77]: '+123.450  '


# = means fill with
In [78]: "{0:<=10.3f}".format(123.45)
Out[78]: '<<<123.450'

In [79]: "{0:A=10.3f}".format(123.45)
Out[79]: 'AAA123.450'

In [80]: "{0:0=10.3f}".format(123.45)
Out[80]: '000123.450'