Deferred evaluation in python

Question

I have heard of deferred evaluation in python (for example here), is it just referring to how lambdas are evaluated by the interpreter only when they are used? Or is this the pr

Answer 1

Dietrich's answer is a good one, but I just want to add that the simplest form of deferred evaluation is the if statement:

if True:
  x = 5
else:
  x = y    # huh? what is y?

This code parses and runs correctly, although the else clause makes no sense - y is undefined. The else clause is only being parsed - so it should be valid Python syntactically. This can be actually used for some simple code:

if stuff:
   print stuff.contents
else:
   print "no stuff"

In a strongly typed language this wouldn't work, because to type stuff.contents requires stuff to be of a certain type that has a contents attribute. In Python, because of the deferred evaluation of the statements in if, this isn't necessarily true. stuff can be None which obviously has no attributes, and the interpreter will just take the else clause without executing the first. Hence this is valid Python and even an idiom, that makes code simpler.

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