Python2中while 1比while True更快
1) bool类是从int类继承而来的
2) True/False 在python2中不是关键字，但是在python3是(True,False,None)
PS > python2 Enthought Canopy Python 2.7.11 | 64-bit | (default, Jun 11 2016, 11:33:47) [MSC v.1500 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> import keyword >>> keyword.kwlist ['and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'exec', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'not', 'or', 'pass', 'print', 'raise', 'return', 'try', 'while', 'with', 'yield'] >>> PS > python3 Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:01:18) [MSC v.1900 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> import keyword >>> keyword.kwlist ['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', ' finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'retu rn', 'try', 'while', 'with', 'yield'] >>>
由于Python2中True/False不是关键字，因此我们可以对其进行任意的赋值
>>> True="test" >>> True 'test' >>> "test"==True True >>> 1==True False >>>
3) 由于bool是继承自int的子类，因此为了保证向下兼容性，在进行算术运算中，True/False会被当作int值来执行
>>> del True >>> True+True 2 >>> False + False 0 >>>
4)While 1 比While True快
import timeit def while_one(): i=0 while 1: i+=1 if i==10000000: break def while_true(): i=0 while True: i+=1 if i==10000000: break if __name__=="__main__": t1=timeit.timeit(while_one,"from __main__ import while_one",number=3) t2=timeit.timeit(while_true,"from __main__ import while_true", number=3) #t1=timeit.timeit(while_one,"from __main__ import while_one",number=3) print "while one : %s \nwhile true: %s " % (t1,t2) while one : 1.16112167105 while true: 1.66502957924
原因就是前提中提到的关键字的问题。由于Python2中，True/False不是关键字，因此我们可以对其进行任意的赋值，这就导致程序在每次循环时都需要对True/False的值进行检查；而对于1，则被程序进行了优化，而后不会再进行检查。
我们可以通过dis模块来查看while_one和while_true的字节码
import dis def while_one(): while 1: pass def while_true(): while True: pass if __name__=="__main__": print "while_one \n" dis.dis(while_one) print "while_true \n" dis.dis(while_true)
结果：
while_one 4 0 SETUP_LOOP 4 (to 7) 5 >> 3 JUMP_ABSOLUTE 3 6 POP_BLOCK >> 7 LOAD_CONST 0 (None) 10 RETURN_VALUE while_true 8 0 SETUP_LOOP 10 (to 13) >> 3 LOAD_GLOBAL 0 (True) 6 POP_JUMP_IF_FALSE 12 9 9 JUMP_ABSOLUTE 3 >> 12 POP_BLOCK >> 13 LOAD_CONST 0 (None) 16 RETURN_VALUE
可以看出，正如上面所讲到的，在while True的时候，字节码中多出了几行语句，正是这几行语句进行了True值的检查
而在Python3中，由于True/False已经是关键字了，不允许进行重新赋值，因此，其执行结果与while 1不再有区别
PS > python3
Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:01:18) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from test_while import * >>> t1=timeit.timeit(while_one,"from __main__ import while_one",number=3) >>> t2=timeit.timeit(while_true,"from __main__ import while_true", number=3) >>> t1 3.1002996925072743 >>> t2 3.077654023474544
5) if x==True or if x:
后者比前者快
#! python2 #-*- coding:utf-8 -*- import timeit def if_x_eq_true(): x=True if x==True: pass def if_x(): x=True if x: pass if __name__=="__main__": t1=timeit.timeit(if_x_eq_true,"from __main__ import if_x_eq_true", number=1000000) t2=timeit.timeit(if_x,"from __main__ import if_x",number=1000000) print "if_x_eq_true: %s \n if_x: %s" % (t1,t2) if_x_eq_true: 0.186029813246 if_x: 0.120894725822
字节码：
import dis def if_x_eq_true(): x=True if x==True: pass def if_x(): x=True if x: pass if __name__=="__main__": print "if_x_eq_true \n" dis.dis(if_x_eq_true) print "if_x \n" dis.dis(if_x)
结果
if_x_eq_true 4 0 LOAD_GLOBAL 0 (True) 3 STORE_FAST 0 (x) 5 6 LOAD_FAST 0 (x) 9 LOAD_GLOBAL 0 (True) 12 COMPARE_OP 2 (==) 15 POP_JUMP_IF_FALSE 21 6 18 JUMP_FORWARD 0 (to 21) >> 21 LOAD_CONST 0 (None) 24 RETURN_VALUE if_x 9 0 LOAD_GLOBAL 0 (True) 3 STORE_FAST 0 (x) 10 6 LOAD_FAST 0 (x) 9 POP_JUMP_IF_FALSE 15 11 12 JUMP_FORWARD 0 (to 15) >> 15 LOAD_CONST 0 (None) 18 RETURN_VALUE
