为什么Python eval
在函数内不起作用?相同的eval(compile(cmd))
代码在全局环境中有效,但在foo
函数内不起作用。
简单示例:
fn = '/tmp/tmp'
mode = 'single'
def foo(cmd, fn, mode):
eval(compile(cmd, fn, mode)) # <<< this does not work
print 'foo: cmd=', cmd
print 'foo: x=', x
cmd = "x = 1"
eval(compile(cmd, fn, mode)) # <<< this works
print 'global scope: cmd=', cmd
print 'global scope: x=', x
del(x)
foo('x = 9', fn, mode)
这是输出和错误消息:
global scope: cmd= x = 1
global scope: x= 1
foo: cmd= x = 9
foo: x=
Traceback (most recent call last):
File "ctest.py", line 20, in <module>
foo('x = 9', fn, mode)
File "ctest.py", line 12, in foo
print 'foo: x=', x
NameError: global name 'x' is not defined
答案 0 :(得分:4)
在您的函数中,执行确实有效,但x
最终在locals()
中,然后print
语句尝试在x
中找到globals()
并且所以提出了NameError
。
fn = '/tmp/tmp'
mode = 'single'
def foo(cmd, fn, mode):
eval(compile(cmd, fn, mode))
print 'locals:', locals()
print 'foo: cmd=', cmd
print 'foo: x=', locals()['x']
cmd = "x = 1"
eval(compile(cmd, fn, mode))
print 'global scope: cmd=', cmd
print 'global scope: x=', x
del(x)
foo('x = 9', fn, mode)
输出:
global scope: cmd= x = 1
global scope: x= 1
locals: {'x': 9, 'cmd': 'x = 9', 'mode': 'single', 'fn': '/tmp/tmp'}
foo: cmd= x = 9
foo: x= 9