python 在调用时计算默认值
大家都知道python的默认值是在函数定义时计算出来的, 也就是说默认值只会计算一次, 之后函数调用时, 如果参数没有给出,同一个值会赋值给变量, 这会导致, 如果我们想要一个list默认值, 新手通常这么写:
def foo(a=[]):
a.append(3)
print a
其实是错误的,两次调用会这样的结果:
其实应该这么写
def baz(a=None):
a = a or []
a.append(3)
print a
两次调用输出以下结果:
这样好挫啊, 搞的虽然有默认值用法,但是我们却需要写的和js,lua一样, 我们不能像c++一样, 在函数运行时每次执行默认值么.
用decorator可以模拟下
import functools
import copy
def compute_default_value_for_each_call(func):
defaults = func.__defaults__
if not defaults:
return None
defaults = tuple(copy.copy(x) for x in defaults)
@functools.wraps(func)
def wrapper(*args, **kwargs):
if func.__defaults__ != defaults:
func.__defaults__ = tuple(copy.copy(y) for y in defaults)
return func(*args, **kwargs)
return wrapper
@compute_default_value_for_each_call
def foo(b, a=[]):
if b:
a.append(3)
return a
import timeit
这样两次调用foo(1), 结果为:
这个decorator有对未修改默认参数值做优化, 在我们不对默认值修改的情况下(比如打印变量a的内容), 性能有很大提升:
@compute_default_value_for_each_call
def foo(b, a=[]):
if b:
a.append(3)
return a
def foo2(b, a=None):
a = a or []
if b:
a.append(3)
return a
import timeit
print timeit.timeit('foo(1)', setup='from __main__ import foo')
print timeit.timeit('foo(0)', setup='from __main__ import foo')
print timeit.timeit('foo2(1)', setup='from __main__ import foo2')
print timeit.timeit('foo2(0)', setup='from __main__ import foo2')
执行结果(调用1000000次的总时间)
4.32704997063
0.630109071732
0.445858955383
0.26370882988
性能上还过得去....
觉得这种方法性能低的同学, 还可以使用ast模块, 直接修改函数源代码, 达到a = a or []这样的性能. 但是实现起来略麻烦, 有机会我再试试.
====================================================
修改函数代码版本如下:
import inspect
import dis
import ast
import re
import traceback
import timeit
def compute_default_value_for_each_call(func):
source = inspect.getsource(func)
m = re.search(r'^\s+', source)
if m:
m.group(0)
n = len(m.group(0))
lines = for line in str.splitlines(source)]
source = '\n'.join(lines)
root = ast.parse(source)
root.body.decorator_list.pop()
arg_names = .args.args]
default_nodes = root.body.args.defaults
arg_names = arg_names
body = root.body.body
n = len(arg_names)
for i in reversed(xrange(n)):
arg_name = arg_names
default_node = default_nodes
lineno = default_node.lineno
col_offset = default_node.col_offset
body.insert(0, ast.Assign(targets=[ast.Name(id=arg_name, ctx=ast.Store(),
lineno=lineno, col_offset=col_offset)],
value=ast.BoolOp(op=ast.Or(), lineno=lineno, col_offset=col_offset,
values=[ast.Name(id=arg_name, ctx=ast.Load(), lineno=lineno, col_offset=col_offset),
default_node]),
lineno=lineno, col_offset=col_offset))
root.body.args.defaults = [ast.Name(id='None', ctx=ast.Load(), lineno=old.lineno, col_offset=old.col_offset)
for old in default_nodes]
root.body.body = body
l = {}
exec compile(root, '', mode='exec') in globals(), l
func = l
return func
def root2():
def main():
@compute_default_value_for_each_call
def root(a=([])):
a.append(3)
return a
print root()
print root()
main()
print 'used in local function'
root2()
def foo():
return 42
bar_count = 0
def bar():
global bar_count
if bar_count:
raise RuntimeError
bar_count += 1
@compute_default_value_for_each_call
def f1(a=foo()):
return a
def f2(a=None):
a = a or foo()
return a
@compute_default_value_for_each_call
def f3(a=bar()):
return a
def f4(a=None):
a = a or bar()
return a
print 'f1:'
dis.dis(f1)
print 'f2:'
dis.dis(f2)
print 'f2 running time:'
print timeit.timeit('f2', setup='from __main__ import f2')
print 'f1 running time:'
print timeit.timeit('f1', setup='from __main__ import f1')
print 'f2 running time:'
print timeit.timeit('f2', setup='from __main__ import f2')
print 'f3:'
dis.dis(f3)
print 'f4:'
dis.dis(f4)
try:
f3()
except:
print traceback.format_exc()
try:
f4()
except:
print traceback.format_exc()
输出:
used in local function
f1:
2 0 LOAD_FAST 0 (a)
3 JUMP_IF_TRUE_OR_POP 12
6 LOAD_GLOBAL 0 (foo)
9 CALL_FUNCTION 0
>> 12 STORE_FAST 0 (a)
3 15 LOAD_FAST 0 (a)
18 RETURN_VALUE
f2:
83 0 LOAD_FAST 0 (a)
3 JUMP_IF_TRUE_OR_POP 12
6 LOAD_GLOBAL 0 (foo)
9 CALL_FUNCTION 0
>> 12 STORE_FAST 0 (a)
84 15 LOAD_FAST 0 (a)
18 RETURN_VALUE
f2 running time:
0.0288169384003
f1 running time:
0.0251071453094
f2 running time:
0.025267124176
f3:
2 0 LOAD_FAST 0 (a)
3 JUMP_IF_TRUE_OR_POP 12
6 LOAD_GLOBAL 0 (bar)
9 CALL_FUNCTION 0
>> 12 STORE_FAST 0 (a)
3 15 LOAD_FAST 0 (a)
18 RETURN_VALUE
f4:
93 0 LOAD_FAST 0 (a)
3 JUMP_IF_TRUE_OR_POP 12
6 LOAD_GLOBAL 0 (bar)
9 CALL_FUNCTION 0
>> 12 STORE_FAST 0 (a)
94 15 LOAD_FAST 0 (a)
18 RETURN_VALUE
Traceback (most recent call last):
File "./b/b.py", line 114, in
f3()
File "", line 2, in f3
File "./b/b.py", line 73, in bar
raise RuntimeError
RuntimeError
Traceback (most recent call last):
File "./b/b.py", line 118, in
f4()
File "./b/b.py", line 93, in f4
a = a or bar()
File "./b/b.py", line 73, in bar
raise RuntimeError
RuntimeError
可以看到性能完全一样了, 在调试信息也还不错
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