Python base clase method call: unexpected behavior

Why does str(A()) seemingly call A.__repr__() and not dict.__str__() in the example below?

class A(dict):
    def __repr__(self):
        return 'repr(A)'
    def __str__(self):
        return dict.__str__(self)

class B(dict):
    def __str__(self):
        return dict.__str__(self)

print 'call: repr(A)  expect: repr(A)  get:', repr(A())   # works
print 'call: str(A)   expect: {}       get:', str(A())    # does not work
print 'call: str(B)   expect: {}       get:', str(B())    # works

Output:

call: repr(A)  expect: repr(A)  get: repr(A)
call: str(A)   expect: {}       get: repr(A)
call: str(B)   expect: {}       get: {}

From stackoverflow

• str(A()) does call __str__, in turn calling dict.__str__().

  It is dict.__str__() that returns the value repr(A).

• I have modified the code to clear things out:

  class A(dict):
     def __repr__(self):
        print "repr of A called",
        return 'repr(A)'
     def __str__(self):
        print "str of A called",
        return dict.__str__(self)
  
  class B(dict):
     def __str__(self):
        print "str of B called",
        return dict.__str__(self)
  

  And the output is:

  >>> print 'call: repr(A)  expect: repr(A)  get:', repr(A())
  call: repr(A)  expect: repr(A)  get: repr of A called repr(A)
  >>> print 'call: str(A)   expect: {}       get:', str(A())
  call: str(A)   expect: {}       get: str of A called repr of A called repr(A)
  >>> print 'call: str(B)   expect: {}       get:', str(B())
  call: str(B)   expect: {}       get: str of B called {}
  

  Meaning that str function calls the repr function automatically. And since it was redefined with class A, it returns the 'unexpected' value.

• Hi,

  I have posted a workaround solution to it. Check it out ... you might find it useful: http://blog.teltub.com/2009/10/workaround-solution-to-python-strrepr.html

  P.S. Read the original post where I introduced the issue as well ... There problem is the unexpected behavior that catches you by surprise ...