Date and string format in python

在 Python 中生成字符串有 3 种方式，现在比较建议的是使用 f-str 的方式，官方说性能更加好。本文主要 介绍在格式化时有哪些格式可以选择。

• String
  • %
  • format
• f-str
  • 进制转化
  • 对齐
• benchmark
• time format
• Reference

String

%

a = 42
b = "%d" % a

c = "answer to everything"
d = "%s %s" % (b, c)

format

a = "{} {}"
b = 1
c = 2
d = a.format(b, c)

a = "{bb} {cc}"
d = a.format(bb=b, cc=c)

person = {"first": "zhang", "last": "san"}
d = "{p[first]}-{p[last]}".format(p=person)

a = "{1} {0}"
d = a.format(b, c)  # 交换位置

f-str

基本上所有的 format 支持得做法 f-str 都是支持的。这里展开介绍下具体的格式。

进制转化

a = 13
print(f"{a:o}")
print(f"{a:b}")
print(f"{a:x}")
print(f"{a:d}")

b = 88.3998128733

print(f"{b:.4f}")
print(f"{b:.4e}")  # 科学技术法

对齐

a = 13

print(f"{a:04}")   # 默认填充左边

# 自定义填充字符
print(f"{a:_>04}") # 填充左边
print(f"{a:_<04}") # 填充右边
print(f"{a:_^04}") # 填充左右

benchmark

python -m timeit "a=1;b=f'{a}'"
# 2000000 loops, best of 5: 102 nsec per loop

python -m timeit "a=1;b='{a}'.format(a=a)"
# 1000000 loops, best of 5: 336 nsec per loop

可以看出 f-str 比 format 的性能还是高不少。

time format

from datetime import datetime as dt

now = dt.now()

format = "%Y-%m-%d %H:%M:%S %a"

print(now.strftime(format))

Code	Example	Description
%a	Sun	Weekday as locale’s abbreviated name.
%A	Sunday	Weekday as locale’s full name.
%w	0	Weekday as a decimal number, where 0 is Sunday and 6 is Saturday.
%d	08	Day of the month as a zero-padded decimal number.
%-	8	Day of the month as a decimal number. (Platform specific)
%b	Sep	Month as locale’s abbreviated name.
%B	September	Month as locale’s full name.
%m	09	Month as a zero-padded decimal number.
%-	9	Month as a decimal number. (Platform specific)
%y	13	Year without century as a zero-padded decimal number.
%Y	2013	Year with century as a decimal number.
%H	07	Hour (24-hour clock) as a zero-padded decimal number.
%-H	7	Hour (24-hour clock) as a decimal number. (Platform specific)
%I	07	Hour (12-hour clock) as a zero-padded decimal number.
%-		7	Hour (12-hour clock) as a decimal number. (Platform specific)
%p	AM	Locale’s equivalent of either AM or PM.
%M	06	Minute as a zero-padded decimal number.
%-M	6	Minute as a decimal number. (Platform specific)
%S	05	Second as a zero-padded decimal number.
%-S	5	Second as a decimal number. (Platform specific)
%f	000000	Microsecond as a decimal number, zero-padded on the left.
%z	+0000	UTC offset in the form ±HHMM[SS[.ffffff]] (empty string if the object is naive).
%Z	UTC	Time zone name (empty string if the object is naive).
%j	251	Day of the year as a zero-padded decimal number.
%-j	251	Day of the year as a decimal number. (Platform specific)
%U	36	Week number of the year (Sunday as the first day of the week) as a zero padded decimal number. All days in a new year preceding the first Sunday are considered to be in week 0.
%W	35	Week number of the year (Monday as the first day of the week) as a decimal number. All days in a new year preceding the first Monday are considered to be in week 0.
%c	Sun Sep 8 07:06:05 2013 Locale’s appropriate date and time representation.
%x	09/08/13	Locale’s appropriate date representation.
%X	07:06:05	Locale’s appropriate time representation.
%%	%	A literal ‘%’ character.

Reference

• PyFormat.info
• Strftime.org