A detailed description of string formatting str. format in Python, pythonstr. format

A detailed description of string formatting str. format in Python, pythonstr. format

Preface

Python 2.6 adds a new string formatting method:str.format(). Its basic syntax is to replace the previous % through {} And ..

Placeholder syntax during formatting:

replacement_field ::= "{" [field_name] ["!" conversion] [":" format_spec] "}"

"Ing" rule

Pass location

str.format() You can accept unlimited parameters, and the positions can be in different order:

>>> "{0} {1}".format("hello", "world")'hello world'>>> "{} {}".format("hello", "world")'hello world'>>> "{1} {0} {1}".format("hello", "world")'world hello world'

Keyword Parameters

When using key parameters, the parameter name must be provided in the string:

>>> "I am {name}, age is {age}".format(name="huoty", age=18)'I am huoty, age is 18'>>> user = {"name": "huoty", "age": 18}>>> "I am {name}, age is {age}".format(**user)'I am huoty, age is 18'

Object Attributes

str.format() You can directly read user attributes:

>>> class User(object):...  def __init__(self, name, age):...   self.name = name...   self.age = age...   ...  def __str__(self):...   return "{self.name}({self.age})".format(self=self)...  ...  def __repr__(self):...   return self.__str__()...  ...>>> user = User("huoty", 18)>>> userhuoty(18)>>> "I am {user.name}, age is {user.age}".format(user=user)'I am huoty, age is 18'

Subscript

You can use subscript to access elements within the string to be formatted:

>>> names, ages = ["huoty", "esenich", "anan"], [18, 16, 8]>>> "I am {0[0]}, age is {1[2]}".format(names, ages)'I am huoty, age is 8'>>> users = {"names": ["huoty", "esenich", "anan"], "ages": [18, 16, 8]}>>> "I am {names[0]}, age is {ages[0]}".format(**users)

Specify Conversion

You can specify the Conversion Type of the string:

 conversion ::= "r" | "s" | "a"

"! R "corresponds to repr ();"! S "corresponding to str ();"! A "corresponds to ascii (). Example:

>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')"repr() shows quotes: 'test1'; str() doesn't: test2"

Format qualifier

Fill and align

Fill is often used together with alignment. ^, <,> Indicates the center, left, and right alignment, respectively, with width and padding after the sign. Only one character can be entered. If this parameter is not specified, spaces are used by default.

>>> "{:>8}".format("181716")' 181716'>>> "{:0>8}".format("181716")'00181716'>>> "{:->8}".format("181716")'--181716'>>> "{:-<8}".format("181716")'181716--'>>> "{:-^8}".format("181716")'-181716-'>>> "{:-<25}>".format("Here ")'Here -------------------->'

Floating point Precision

Use f to indicate the floating point type, and you can add precision control on the front side:

>>> "[ {:.2f} ]".format(321.33345)'[ 321.33 ]'>>> "[ {:.1f} ]".format(321.33345)'[ 321.3 ]'>>> "[ {:.4f} ]".format(321.33345)'[ 321.3335 ]'>>> "[ {:.4f} ]".format(321)'[ 321.0000 ]'

You can also specify a symbol for a floating point number. + indicates displaying + before a positive number and-before a negative number. (Space) indicates adding a space before a positive number and adding-before a negative number -; -Consistent with when nothing is added ({: f:

>>> '{:+f}; {:+f}'.format(3.141592657, -3.141592657)'+3.141593; -3.141593'>>> '{: f}; {: f}'.format(3.141592657, -3.141592657)' 3.141593; -3.141593'>>> '{:f}; {:f}'.format(3.141592657, -3.141592657)'3.141593; -3.141593'>>> '{:-f}; {:-f}'.format(3.141592657, -3.141592657)'3.141593; -3.141593'>>> '{:+.4f}; {:+.4f}'.format(3.141592657, -3.141592657)'+3.1416; -3.1416'

Specify the base

>>> "int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}".format(18)'int: 18; hex: 12; oct: 22; bin: 10010'>>> "int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}".format(18)'int: 18; hex: 0x12; oct: 0o22; bin: 0b10010'

Thousands Separator

You can use "," as the thousands separator:

>>> '{:,}'.format(1234567890)'1,234,567,890'

Percent display

>>> "progress: {:.2%}".format(19.88/22)'progress: 90.36%'

In fact, the format supports more types:

type ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

Other skills

Placeholder nesting

Sometimes placeholder Nesting is useful:

>>> '{0:{fill}{align}16}'.format("hello", fill='*', align='^')'*****hello******'>>>>>> for num in range(5,12):...  for base in "dXob":...   print("{0:{width}{base}}".format(num, base=base, width=5), end=' ')...  print()...  ... 5  5  5 101 6  6  6 110 7  7  7 111 8  8 10 1000 9  9 11 1001 10  A 12 1010 11  B 13 1011

Used as a function

You can call this function as a function without specifying the formatting parameter:

>>> email_f = "Your email address was {email}".format>>> print(email_f(email="suodhuoty@gmail.com"))Your email address was sudohuoty@gmail.com

Escape braces

When braces are required in a string, they can be escaped using braces:

>>> " The {} set is often represented as { {0} } ".format("empty")' The empty set is often represented as {0} '

Summary

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