How do I check if a variable exists

Question

I want to check if a variable exists  Now I m doing something like this   try     myVar except NameError       Do something    Are there other ways without exceptions

User · Answer

I will assume that the test is going to be used in a function, similar to user97370's answer. I don't like that answer because it pollutes the global namespace. One way to fix it is to use a class instead:

class InitMyVariable(object):
  my_variable = None

def __call__(self):
  if self.my_variable is None:
   self.my_variable = ...

I don't like this, because it complicates the code and opens up questions such as, should this confirm to the Singleton programming pattern? Fortunately, Python has allowed functions to have attributes for a while, which gives us this simple solution:

def InitMyVariable():
  if InitMyVariable.my_variable is None:
    InitMyVariable.my_variable = ...
InitMyVariable.my_variable = None

User · Answer

This was my scenario  for i in generate numbers        do something i    Use the last i   I can   t easily determine the length of the iterable  and that means that i may or may not exist depending on whether the iterable produces an empty sequence  If I want to use the last i of the iterable  an i that doesn   t exist for an empty sequence  I can do one of two things  i   None    Declare the variable  for i in generate numbers        do something i  use last i   or for i in generate numbers        do something i  try      use last i  except UnboundLocalError      pass    i didn   t exist because sequence was empty   The first solution may be problematic because I can   t tell  depending on the sequence values  whether i was the last element  The second solution is more accurate in that respect

User · Answer

To check the existence of a local variable   if  myVar  in locals        myVar exists    To check the existence of a global variable   if  myVar  in globals        myVar exists    To check if an object has an attribute   if hasattr obj   attr name        obj attr name exists

User · Answer

catch is called except in Python  other than that it s fine for such simple cases  There s the AttributeError that can be used to check if an object has an attribute

User · Answer

The use of variables that have yet to been defined or set  implicitly or explicitly  is often a bad thing in any language  since it tends to indicate that the logic of the program hasn t been thought through properly  and is likely to result in unpredictable behaviour  If you need to do it in Python  the following trick  which is similar to yours  will ensure that a variable has some value before use  try      myVar except NameError      myVar   None        or some other default value     Now you re free to use myVar without Python complaining   However  I m still not convinced that s a good idea - in my opinion  you should try to refactor your code so that this situation does not occur

User · Answer

I created a custom function  def exists var        return var in globals    Then the call the function like follows replacing variable name with the variable you want to check  exists  quot variable name quot    Will return True or False

User · Answer

Like so  def no var        quot give var as a string  quote it like  var   quot      assert var not in vars        assert var not in globals        assert var not in vars   builtins         import keyword     assert var not in keyword kwlist   Then later  no  foo   foo         If your new variable foo is not safe to use  you ll get an AssertionError exception which will point to the line that failed  and then you will know better  Here is the obvious contrived self-reference  no  no    --------------------------------------------------------------------------- AssertionError                            Traceback  most recent call last   lt ipython-input-88-d14ecc6b025a gt  in  lt module gt  ---- gt  1 no  no     lt ipython-input-86-888a9df72be0 gt  in no var        2      quot give var as a string  quote it  quot        3     assert  var not in vars    ---- gt  4     assert  var not in globals          5     assert  var not in vars   builtins           6     import keyword  AssertionError

User · Answer

Using try except is the best way to test for a variable s existence  But there s almost certainly a better way of doing whatever it is you re doing than setting testing global variables   For example  if you want to initialize a module-level variable the first time you call some function  you re better off with code something like this   my variable   None  def InitMyVariable      global my variable   if my variable is None      my variable

User · Answer

A way that often works well for handling this kind of situation is to not explicitly check if the variable exists but just go ahead and wrap the first usage of the possibly non-existing variable in a try except NameError     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

User · Answer

A simple way is to initialize it at first saying myVar   None  Then later on      if myVar is not None        Do something

User · Answer

for objects modules  you can also    var  in dir obj    For example     gt  gt  gt  class Something object           pass      gt  gt  gt  c   Something    gt  gt  gt  c a   1  gt  gt  gt   a  in dir c  True  gt  gt  gt   b  in dir c  False

User · Answer

Short variant  my var   some value if  my var  not in globals   else my var

[python] How do I check if a variable exists?

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