判断Python中是否定义了变量
如何知道运行时代码中的特定位置是否已设置变量？这并不总是显而易见的，因为 (1) 可以有条件地设置变量，并且 (2) 可以有条件地删除变量。我正在寻找类似 Perl 中的 define() 或 PHP 中的 isset() 或 Ruby 中的 define? 的内容。
if condition:
a = 42
# is "a" defined here?
if other_condition:
del a
# is "a" defined here?
How do you know whether a variable has been set at a particular place in the code at runtime? This is not always obvious because (1) the variable could be conditionally set, and (2) the variable could be conditionally deleted. I'm looking for something like defined() in Perl or isset() in PHP or defined? in Ruby.
if condition:
a = 42
# is "a" defined here?
if other_condition:
del a
# is "a" defined here?
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vars() 中的 'a' 或 globals() 中的 'a'如果你想学究气，你也可以检查内置函数
vars(__builtins__) 中的“a”'a' in vars() or 'a' in globals()if you want to be pedantic, you can check the builtins too
'a' in vars(__builtins__)我认为最好避免这种情况。写起来更干净、更清晰：
I think it's better to avoid the situation. It's cleaner and clearer to write:
对于这种特殊情况，最好使用
a = None而不是del a。这将减少分配给对象a（如果有）的引用计数，并且在未定义a时不会失败。请注意，del 语句不会直接调用对象的析构函数，而是将其与变量解除绑定。当引用计数变为零时，调用对象的析构函数。For this particular case it's better to do
a = Noneinstead ofdel a. This will decrement reference count to objectawas (if any) assigned to and won't fail whenais not defined. Note, thatdelstatement doesn't call destructor of an object directly, but unbind it from variable. Destructor of object is called when reference count became zero.可能需要这样做的一种可能情况是：
如果您使用
finally块来关闭连接，但在try块中，程序将通过sys.exit() 退出在定义连接之前。在这种情况下，将调用finally块，并且连接关闭语句将失败，因为没有创建连接。One possible situation where this might be needed:
If you are using
finallyblock to close connections but in thetryblock, the program exits withsys.exit()before the connection is defined. In this case, thefinallyblock will be called and the connection closing statement will fail since no connection was created.