Detailed introduction to Python string formatting

This article brings you a detailed introduction to the formatting of Python strings. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. .

I believe that many people use the "%s" % v syntax when formatting strings. PEP 3101 proposes a more advanced formatting method str.format() and becomes the standard in Python 3. To replace the old %s formatting syntax, CPython has implemented this method since 2.6 (other interpreters have not verified it).

format()

The new format() method is actually more like a simplified version of the template engine (Template Engine), with very rich functions.

The replacement variable in the template is surrounded by {} and divided into two parts by :, the second half of which, format_spec, will be discussed separately later.

The first half has three uses:

• empty
• The number representing the position
• The identifier of the keyword

This is consistent with the parameter category of function calls

print("{} {}".format("Hello", "World"))
# 等同于以下几种
print("{0} {1}".format("Hello", "World"))
print("{hello} {world}".format(hello="Hello", world="World"))
print("{0}{1}{0}".format("H", "e"))

# Hello World
# Hello World
# Hello World
# HeH

In addition, just like the unpacking of function parameters, the unpacking operation can also be used directly in format()

print("{author}.{city}".format(**{"author": "Miracle", "city": "上海"}))
print("{} {}".format(*["Miracle", "上海"]))

Miracle.上海
Miracle 上海

In the template, you can also obtain the attributes or values ​​​​in the variable through .identifier and [key] (it should be noted that "{}{}" is equivalent to "{0}{1}")

data = {'author': 'Miracle', 'like': 'papapa'}
print("Author: {0[author]}, Like: {0[like]}".format(data))
langs = ["Python", "Ruby"]
print("{0[0]} vs {0[1]}".format(langs))

print("\n====\nHelp(format):{.__doc__}".format(str.format))

# Name: Python, Score: 100
# Python vs Ruby

# ====
# Help(format):
#  S.format(*args, **kwargs) -> str

Forced conversion, you can force the replaced variable through ! r|s|a

• "{!r}" Call repr() on the variable
• "{ !s}" Call str() on the variable
• "{!a}" Call ascii() on the variable

The part after the colon defines the output style

align represents the alignment direction, usually used in conjunction with width, and fill is the filling character (default is blank):

for align, text in zip("<^>", ["left", "center", "right"]):
   # 务必看懂这句话
   print("{:{fill}{align}16}".format(text, fill=align, align=align))

print("{:0=10}".format(100)) # = 只允许数字

# left<<<<<<<<<<<<
# ^^^^^center^^^^^
# >>>>>>>>>>>right
# 0000000100

At the same time, it can be seen that {} can be nested in the style setting, but it must pass the keyword Specified, and can only be nested one level.

The next step is the symbol style: |-|' ' respectively specifies whether the number requires a mandatory symbol (the space means that it will not be displayed when the number is positive but one space will be reserved)

print("{0:+}\n{1:-}\n{0: }".format(3.14, -3.14))

# +3.14
# -3.14
# 3.14

Use Whether a prefix symbol is needed to represent numbers in special formats (binary, hexadecimal, etc.)

Comma is also used to represent numbers whether they need to be separated at the thousands place

0 is equivalent to the previous {:0=} is right-aligned and filled with 0s

print("Binary: {0:b} => {0:#b}".format(3))
print("Large Number: {0:} => {0:,}".format(1.25e6))
print("Padding: {0:16} => {0:016}".format(3))

# Binary: 11 => 0b11
# Large Number: 1250000.0 => 1,250,000.0
# Padding:                3 => 0000000000000003

Finally, Xiaopang will introduce to you the familiar decimal point precision issues, .n and formatting types.

Only some examples are given here, please refer to the documentation for details:

from math import pi
print("pi = {pi:.2}, also = {pi:.7}".format(pi=pi))

# pi = 3.1, also = 3.141593

Integer

for t in "b c d #o #x #X n".split():
   print("Type {0:>2} of {1} shows: {1:{t}}".format(t, 97, t=t))

# Type  b of 97 shows: 1100001
# Type  c of 97 shows: a
# Type  d of 97 shows: 97
# Type #o of 97 shows: 0o141
# Type #x of 97 shows: 0x61
# Type #X of 97 shows: 0X61
# Type  n of 97 shows: 97

Float

for t, n in zip("eEfFgGn%", [12345, 12345, 1.3, 1.3, 1, 2, 3.14, 0.985]):
   print("Type {} shows: {:.2{t}}".format(t, n, t=t))

# Type e shows: 1.23e+04
# Type E shows: 1.23E+04
# Type f shows: 1.30
# Type F shows: 1.30
# Type g shows: 1
# Type G shows: 2
# Type n shows: 3.1
# Type % shows: 98.50%

String (default)

try:
   print("{:s}".format(123))
except:
   print("{}".format(456))

# 456

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