Proper way to assert type of variable in Python

Question

In using a function  I wish to ensure that the type of the variables are as expected  How to do it right   Here is an example fake function trying to do just this before going on with its role   def my print begin  text  end          Print  text  in UPPER between  begin  and  end  in lower              for i in  begin  text  end           assert isinstance i  str    Input variables should be strings      out   begin lower     text upper     end lower       print out  def test           Put your test cases here               assert my print  asdf    fssfpoie    fsodf       assert not my print  fasdf   33   adfas       print  All tests passed   test     Is assert the right approach  Should I use try except instead   Also  my assert set of tests does not seem to work properly  S  Thanks pythoneers

User · Accepted Answer

The isinstance built-in is the preferred way if you really must  but even better is to remember Python s motto   it s easier to ask forgiveness than permission  -    It was actually Grace Murray Hopper s favorite motto -    I e    def my print text  begin  end        Print  text  in UPPER between  begin  and  end  in lower      try        print begin lower     text upper     end lower       except  AttributeError  TypeError         raise AssertionError  Input variables should be strings     This  BTW  lets the function work just fine on Unicode strings -- without any extra effort -

User · Answer

Doing type     is effectively equivalent to str and types StringType  so type        str    types StringType will evaluate to  True   Note that Unicode strings which only contain ASCII will fail if checking types in this way  so you may want to do something like assert type s  in  str  unicode  or assert isinstance obj  basestring   the latter of which was suggested in the comments by 007Brendan and is probably preferred   isinstance   is useful if you want to ask whether an object is an instance of a class  e g   class MyClass  pass  print isinstance MyClass    MyClass    - gt  True print isinstance MyClass  MyClass      - gt  TypeError exception   But for basic types  e g  str  unicode  int  float  long etc asking type var     TYPE will work OK

User · Answer

You might want to try this example for version 2 6 of Python   def my print text  begin  end        Print text in UPPER between  begin  and  end  in lower       for obj in  text  begin  end           assert isinstance obj  str    Argument of wrong type       print begin lower     text upper     end lower     However  have you considered letting the function fail naturally instead

[python] Proper way to assert type of variable in Python

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