eval和列表解析的一处陷阱

>>> def f():
    a=1
    return [i+a for i in range(3)]

>>> f()
[1, 2, 3]
>>> def f():
    a=1
    return [i+eval('a') for i in range(3)]

>>> f()
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    f()
  File "<pyshell#12>", line 3, in f
    return [i+eval('a') for i in range(3)]
  File "<pyshell#12>", line 3, in <listcomp>
    return [i+eval('a') for i in range(3)]
  File "<string>", line 1, in <module>
NameError: name 'a' is not defined

看看locals()是什么:

>>> def f():
    a=1
    return [print(locals()) for i in range(3)]

>>> f()
{'i': 0, '.0': <range_iterator object at 0x0000000003353530>}
{'i': 1, '.0': <range_iterator object at 0x0000000003353530>}
{'i': 2, '.0': <range_iterator object at 0x0000000003353530>}
[None, None, None]

>>> def f():
    a=1
    return [(i+a,print(locals())) for i in range(3)]

>>> f()
{'a': 1, 'i': 0, '.0': <range_iterator object at 0x0000000003353530>}
{'a': 1, 'i': 1, '.0': <range_iterator object at 0x0000000003353530>}
{'a': 1, 'i': 2, '.0': <range_iterator object at 0x0000000003353530>}
[(1, None), (2, None), (3, None)]