处理这种情况的一种方法通常是不显式检查变量是否存在，而是直接将可能不存在的变量的第一次使用包装在try/except NameError中:

# Search for entry.
for x in y:
  if x == 3:
    found = x

# Work with found entry.
try:
  print('Found: {0}'.format(found))
except NameError:
  print('Not found')
else:
  # Handle rest of Found case here
  ...

简短的变体:

my_var = some_value if 'my_var' not in globals() else my_var:

对于对象/模块，也可以

'var' in dir(obj)

例如,

>>> class Something(object):
...     pass
...
>>> c = Something()
>>> c.a = 1
>>> 'a' in dir(c)
True
>>> 'b' in dir(c)
False

查询一个局部变量是否存在。

if 'myVar' in locals():
  # myVar exists.

检查全局变量是否存在:

if 'myVar' in globals():
  # myVar exists.

检查一个对象是否有属性:

if hasattr(obj, 'attr_name'):
  # obj.attr_name exists.

处理这种情况的一种方法通常是不显式检查变量是否存在，而是直接将可能不存在的变量的第一次使用包装在try/except NameError中:

# Search for entry.
for x in y:
  if x == 3:
    found = x

# Work with found entry.
try:
  print('Found: {0}'.format(found))
except NameError:
  print('Not found')
else:
  # Handle rest of Found case here
  ...

像这样:

def no(var):
    "give var as a string (quote it like 'var')"
    assert(var not in vars())
    assert(var not in globals())
    assert(var not in vars(__builtins__))
    import keyword
    assert(var not in keyword.kwlist)

之后:

no('foo')
foo = ....

如果你的新变量foo使用起来不安全，你会得到一个AssertionError异常，它会指向失败的行，然后你就知道了。 这是一个明显做作的自我指涉:

no('no')

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-88-d14ecc6b025a> in <module>
----> 1 no('no')

<ipython-input-86-888a9df72be0> in no(var)
      2     "give var as a string (quote it)"
      3     assert( var not in vars())
----> 4     assert( var not in globals())
      5     assert( var not in vars(__builtins__))
      6     import keyword

AssertionError: