Determine if variable is defined in Python

Question

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

User · Answer

I think it s better to avoid the situation   It s cleaner and clearer to write   a   None if condition      a   42

User · Answer

One possible situation where this might be needed   If you are using finally block to close connections but in the try block  the program exits with sys exit   before the connection is defined  In this case  the finally block will be called and the connection closing statement will fail since no connection was created

User · Answer

try      a   does a exist in the current namespace except NameError      a   10   nope

User · Answer

a  in vars   or  a  in globals    if you want to be pedantic  you can check the builtins too  a  in vars   builtins

User · Answer

try      thevariable except NameError      print  well  it WASN T defined after all    else      print  sure  it was defined

User · Answer

For this particular case it s better to do a   None instead of del a  This will decrement reference count to object a was  if any  assigned to and won t fail when a is not defined  Note  that del statement doesn t call destructor of an object directly  but unbind it from variable  Destructor of object is called when reference count became zero

[python] Determine if variable is defined in Python

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