Detailed introduction to string formatting str.format in Python

Preface

Python has added a new string formatting method in version 2.6: str.format(). Its basic syntax is to replace the previous %. with {} and :.

Placeholder syntax when formatting:

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

"Mapping" rules

By position

str.format() Can accept unlimited parameters, and the positions can be out of order:

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

By keyword parameters

When using key parameters, the parameter name needs to 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'

Through object properties

str.format() User properties can be read directly:

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

By subscript

Elements can be accessed by subscripts inside the string that needs 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 string:

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

Where "!r" corresponds to repr(); "!s" corresponds 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

Padding and Alignment

Padding is often used together with alignment. ^, <, > are respectively centered, left-aligned, and right-aligned, followed by width, and the character followed by : can only be one character. If not specified, it will be filled with spaces by default.

>>> "{:>8}".format("181716")
' 181716'
>>> "{:0>8}".format("181716")
'00181716'
>>> "{:->8}".format("181716")
'--181716'
>>> "{:-<8}".format("181716")
&#39;181716--&#39;
>>> "{:-^8}".format("181716")
&#39;-181716-&#39;
>>> "{:-<25}>".format("Here ")
&#39;Here -------------------->&#39;

Floating point precision

Use f to represent the floating point type, and you can add precision control in front of it ：

>>> "[ {:.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 floating point numbers, + means + will be displayed before positive numbers, and - will be displayed before negative numbers; (space) means before positive numbers Adding a space and adding -;- before a negative number is consistent with adding nothing ({: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 system

>>> "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'

Thousand 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, format Also supports more type symbols:

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

Other tips

Placeholder nesting

Sometimes placeholder nesting is still 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

As When using the function

, you can not specify the format parameters first, but call it as a function in an unnecessary place:

>>> 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 you need to use braces in a string, you can use braces to escape:

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

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