testing – Proper way to assert type of variable in Python

testing – Proper way to assert type of variable in Python

The isinstance built-in is the preferred way if you really must, but even better is to remember Pythons motto: its easier to ask forgiveness than permission!-) (It was actually Grace Murray Hoppers 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!-)

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?

testing – Proper way to assert type of variable in Python

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) # -> True
print isinstance(MyClass, MyClass()) # -> TypeError exception

But for basic types, e.g. str, unicode, int, float, long etc asking type(var) == TYPE will work OK.