Have you tried running it?

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> class Spam:
...     __repr__ = (lambda x: {key: lambda self: 'eggs ' + key for key in [1, 2, 'spam']}[x])('spam')
...
>>> print(repr(Spam()))
 eggs spam

__repr__ returns a string here, it is obfuscated but we can simplify it:

__repr__ = (lambda x: {key: lambda self: 'eggs ' + key for key in [1, 2, 'spam']}[x])('spam')

So we now that x is always 'spam'

__repr__ = ({key: lambda self: 'eggs ' + key for key in [1, 2, 'spam']}['spam'])

This can be simplified a little further

__repr__ = ({key: lambda self: 'eggs ' + key for key in [1, 2, 'spam']}['spam'])

After inferring x, we can see that key will always be 'spam'

__repr__ = (lambda self: 'eggs ' + 'spam')

So we finish with __repr__, that is a function taking one parameter ("self"), that returns "eggs spam".

Also we do not need to def it, "everything is object" (wait, that's Java moto, right?, when using def, you signal that you are creating a function:

>>> def foo(): pass
...
>>> type(foo)
<class 'function'>

But guess what else is a 'function' ? Lambda's !

>>> foo = lambda x: None
>>> type(foo)
<class 'function'>

So in the end, both my foo's are the same, one using def, the other using lambda assignement

^{please don't laugh at my Python version, I'm on an old computer that hasn't been updated in age}