pyCurious

Metaclasses seem to be an obscure, misunderstood area of Python’s object model. Here is a simple example that I’ve used to both understand and explain what a metaclass is a how it can be used.

This code snippet was spawned from a colleague’s query.

“What would be the most pythonic way to override a class type’s string representation?”

Expressed in code he wanted a new-style class type A to behave like

>>> A
<class '__main__.A'>
>>> str(A)
'Hello World'

Unfotunately overriding A’s __str__ method

>>> class A(object):
...  def __str__(self):
...   return "Hello World"
... 
>>> A
<class '__main__.A'>
>>> str(A)
"<class '__main__.A'>"
>>> str(A())
'Hello World'
>>>

only overrides the string representation of instances of A rather than the type A.

The solution to the problem leads us to metaclasses. My colleague’s conundrum can be solved by creating a new-style class with base type and a __str__ method. This acts to override the __str__ method of A’s type rather than instances of A.

>>> class meta_A(type):
...  def __str__(cls):
...   return "Hello World"
... 
>>> class A(object):
...  __metaclass__ = meta_A
... 
>>> A
<class '__main__.A'>
>>> str(A)
'Hello World'
>>> str(A())
'<__main__.A object at 0x10049e710>'

Thus we have a pythonic mechanism for overriding the string representation of a class type.

Note that there is new syntax in Python 3.0 for specifying a metaclass.

>>> class A(metaclass=meta_A):
...  pass
...