确定变量是否在Python中定义

Question

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Easy way to check that variable is defined in python?
How do I check if a variable exists in Python?

如何知道在运行时是否在代码中的特定位置设置了变量？这并不总是显而易见的，因为（1）变量可以有条件地设置，（2）变量可以有条件地删除。我正在寻找Perl中的defined()，或者PHP中的isset()，或者Ruby中的defined?。

if condition:
    a = 42

# is "a" defined here?

if other_condition:
    del a

# is "a" defined here?

Answer 1

我认为最好避免这种情况。写得越来越清楚：

a = None
if condition:
    a = 42

Answer 2

'a' in vars() or 'a' in globals()

如果你想学究，你也可以检查内置
'a' in vars(__builtins__)

Answer 3

try:
    thevariable
except NameError:
    print("well, it WASN'T defined after all!")
else:
    print("sure, it was defined.")