Question

def myFunc( str ):
      print "str=", str
      if str == None:
        print "str is None"
      else:
        print "str is not None, value is:", str

This function is called multiple times in my app with str being None. However sometimes, although str is None, the test fails and it prints:

str=None
str is not None, value is None

How can this happen ?

Answer 1

The string 'None' and the bytestring b'None' will both print out None, but not actually be none. Also, you can have custom classes which override their __str__ methods to return 'None', although they’re actually not None.

Some aesthetic notes: Python guarantees that there’ll only ever be one instance of None, so you should use is instead of ==. Also, you should not name your variable str, as that’s the name of a built-in.

Try this definition:

def myFunc(s):
    if s is None:
        print('str is None')
    else:
        print('str is not None, it is %r of type %s' % (s, type(s).__name__))