Consider the following proof:

Proposition: x + y is odd whenever x and y are integers of opposite parity.

Proof: Let x and y be integers of opposite parity. Without loss of generality, suppose x is even and y is odd. Then there exist integers k and l such that x = 2k and y = 2l + 1, thus, x + y = 2k + 2l + 1 = 2(k + l) + 1, which is odd. ∎

This proof is made half-as-verbose through the use of the phrase "without loss of generality", allowing the writer to trust his readers to mentally fill in the blanks and apply the single given argument to any other cases.

Now, you can bring the same magic to Python, allowing programmers to trust the interpreter to implicitly fill in the blanks and apply the single given block of code to any other cases:

>>> from wlog import *
>>> x = 327
>>> y = 94
>>> with out_loss_of_generality(lambda x, y: x % 2 == 0 and y % 2 == 1):
...     print("x: %d" % x)
...     print("y: %d" % y)
...
x: 94
y: 327

Specifically, wlog.py will try all permutations of binding existing local identifiers to existing values in the current scope until it finds one that satisfies a given predicate. It will rebind the identifiers defined as the lambda's parameters to the satisfying values, then restore their original values once the enclosed block is exited.

What could go wrong?