5年Python功力，总结了10个开发技巧

如何在运行状态查看源代码？

#?demo.py
import?inspect


def?add(x,?y):
????return?x?+?y

print("===================")
print(inspect.getsource(add))

$?python?demo.py
===================
def?add(x,?y):
????return?x?+?y

如何关闭异常自动关联上下文？

try:
????print(1?/?0)
except?Exception?as?exc:
????raise?RuntimeError("Something?bad?happened")

Traceback?(most?recent?call?last):
??File?"demo.py",?line?2,?in?<module>
????print(1?/?0)
ZeroDivisionError:?division?by?zero

During?handling?of?the?above?exception,?another?exception?occurred:

Traceback?(most?recent?call?last):
??File?"demo.py",?line?4,?in?<module>
????raise?RuntimeError("Something?bad?happened")
RuntimeError:?Something?bad?happened

try:
????print(1?/?0)
except?Exception?as?exc:
????raise?RuntimeError("Something?bad?happened")?from?exc

Traceback?(most?recent?call?last):
??File?"demo.py",?line?2,?in?<module>
????print(1?/?0)
ZeroDivisionError:?division?by?zero

The?above?exception?was?the?direct?cause?of?the?following?exception:

Traceback?(most?recent?call?last):
??File?"demo.py",?line?4,?in?<module>
????raise?RuntimeError("Something?bad?happened")?from?exc
RuntimeError:?Something?bad?happened

try:
????print(1?/?0)
except?Exception?as?exc:
????raise?RuntimeError("bad?thing").with_traceback(exc)

$?cat?demo.py
try:
????print(1?/?0)
except?Exception?as?exc:
????raise?RuntimeError("Something?bad?happened")?from?None
$
$?python?demo.py
Traceback?(most?recent?call?last):
??File?"demo.py",?line?4,?in?<module>
????raise?RuntimeError("Something?bad?happened")?from?None
RuntimeError:?Something?bad?happened
(PythonCodingTime)

最快查看包搜索路径的方式

>>>?import?sys
>>>?from?pprint?import?pprint???
>>>?pprint(sys.path)
['',
?'/usr/local/Python3.7/lib/python37.zip',
?'/usr/local/Python3.7/lib/python3.7',
?'/usr/local/Python3.7/lib/python3.7/lib-dynload',
?'/home/wangbm/.local/lib/python3.7/site-packages',
?'/usr/local/Python3.7/lib/python3.7/site-packages']
>>>?

[wangbm@localhost?~]$?python?-c?"print('\n'.join(__import__('sys').path))"

/usr/lib/python2.7/site-packages/pip-18.1-py2.7.egg
/usr/lib/python2.7/site-packages/redis-3.0.1-py2.7.egg
/usr/lib64/python27.zip
/usr/lib64/python2.7
/usr/lib64/python2.7/plat-linux2
/usr/lib64/python2.7/lib-tk
/usr/lib64/python2.7/lib-old
/usr/lib64/python2.7/lib-dynload
/home/wangbm/.local/lib/python2.7/site-packages
/usr/lib64/python2.7/site-packages
/usr/lib64/python2.7/site-packages/gtk-2.0
/usr/lib/python2.7/site-packages

[wangbm@localhost?~]$?python3?-m?site
sys.path?=?[
????'/home/wangbm',
????'/usr/local/Python3.7/lib/python37.zip',
????'/usr/local/Python3.7/lib/python3.7',
????'/usr/local/Python3.7/lib/python3.7/lib-dynload',
????'/home/wangbm/.local/lib/python3.7/site-packages',
????'/usr/local/Python3.7/lib/python3.7/site-packages',
]
USER_BASE:?'/home/wangbm/.local'?(exists)
USER_SITE:?'/home/wangbm/.local/lib/python3.7/site-packages'?(exists)
ENABLE_USER_SITE:?True

将嵌套 for 循环写成单行

list1?=?range(1,3)
list2?=?range(4,6)
list3?=?range(7,9)
for?item1?in?list1:
????for?item2?in?list2:
???????for?item3?in?list3:
???????????print(item1+item2+item3)

from?itertools?import?product
list1?=?range(1,3)
list2?=?range(4,6)
list3?=?range(7,9)
for?item1,item2,item3?in?product(list1,?list2,?list3):
????print(item1+item2+item3)

$?python?demo.py
12
13
13
14
13
14
14
15

如何使用 print 输出日志

>>>?with?open('test.log',?mode='w')?as?f:
...?????print('hello,?python',?file=f,?flush=True)
>>>?exit()

$?cat?test.log
hello,?python

如何快速计算函数运行时间

import?time

start?=?time.time()

#?run?the?function

end?=?time.time()
print(end-start)

import?time
import?timeit

def?run_sleep(second):
????print(second)
????time.sleep(second)

#?只用这一行
print(timeit.timeit(lambda?:run_sleep(2),?number=5))

2
2
2
2
2
10.020059824

利用自带的缓存机制提高效率

@functools.lru_cache(maxsize=None,?typed=False)

• maxsize：最多可以缓存多少个此函数的调用结果，如果为None，则无限制，设置为 2 的幂时，性能最佳
• typed：若为 True，则不同参数类型的调用将分别缓存。

from?functools?import?lru_cache

@lru_cache(None)
def?add(x,?y):
????print("calculating:?%s?+?%s"?%?(x,?y))
????return?x?+?y

print(add(1,?2))
print(add(1,?2))
print(add(2,?3))

calculating:?1?+?2
3
3
calculating:?2?+?3
5

def?fib(n):
????if?n?<?2:
????????return?n
????return?fib(n?-?2)?+?fib(n?-?1)

import?timeit

def?fib(n):
????if?n?<?2:
????????return?n
????return?fib(n?-?2)?+?fib(n?-?1)



print(timeit.timeit(lambda?:fib(40),?number=1))
#?output:?31.2725698948

import?timeit
from?functools?import?lru_cache

@lru_cache(None)
def?fib(n):
????if?n?<?2:
????????return?n
????return?fib(n?-?2)?+?fib(n?-?1)

print(timeit.timeit(lambda?:fib(500),?number=1))
#?output:?0.0004921059880871326

在程序退出前执行代码的技巧

• 如果程序是被你没有处理过的系统信号杀死的，那么注册的函数无法正常执行。
• 如果发生了严重的 Python 内部错误，你注册的函数无法正常执行。
• 如果你手动调用了os._exit()，你注册的函数无法正常执行。

实现类似 defer 的延迟调用

import?"fmt"

func?myfunc()?{
????fmt.Println("B")
}

func?main()?{
????defer?myfunc()
????fmt.Println("A")
}

A
B

import?contextlib

def?callback():
????print('B')

with?contextlib.ExitStack()?as?stack:
????stack.callback(callback)
????print('A')

A
B

如何流式读取数G超大文件

#?一次性读取
with?open("big_file.txt",?"r")?as?fp:
????content?=?fp.read()

def?read_from_file(filename):
????with?open(filename,?"r")?as?fp:
????????yield?fp.readline()

def?read_from_file(filename,?block_size?=?1024?*?8):
????with?open(filename,?"r")?as?fp:
????????while?True:
????????????chunk?=?fp.read(block_size)
????????????if?not?chunk:
????????????????break

????????????yield?chunk

from?functools?import?partial

def?read_from_file(filename,?block_size?=?1024?*?8):
????with?open(filename,?"r")?as?fp:
????????for?chunk?in?iter(partial(fp.read,?block_size),?""):
????????????yield?chunk