[Python-ideas] Introducing where clauses

Steven D'Aprano steve at pearwood.info
Mon Jun 22 13:34:45 CEST 2009

On Mon, 22 Jun 2009 08:40:05 pm Nick Coghlan wrote:
> Lambdas, comprehensions and expressions in general all have limits -
> usually deliberate ones. When one runs up against those limits it is
> a hint that it is time to switch to using multiple statements
> (typically factored out into a function that can be substituted for
> the original inline expression)


> But then, I'll freely confess to not really understanding the
> apparently common obsession with wanting to be able to do everything
> as an expression.

Some things are conceptually a single operation, and those things are 
good to write as a single expression. Before we had sorted(), it was 
uncomfortable to write:

return L

when you wanted a sorted list, because "return a sorted list" is 
conceptually a single operation, even if sorting is non-trivial. The 
solution to this was to write a helper function, which was made 
obsolete when sorted() became a built-in.

The OP's suggestion:

[(f(x), f(x)) for x in some_iterable if f(x) < 2]

is not conceptually a single operation, because producing a list of 
two-tuples containing some value y repeated but only if y is less than 
2 is not conceptually simple. If it were, it would be easy to describe 
the operation with one or two words, instead of the fourteen it took 

I still believe that the right way to solve this is with a pipeline of 
simple operations:

map(lambda obj: (obj, obj),
    filter(lambda y: y < 2, 
    map(f, some_iterable)))

Re-write with temporary variables, itertools, and generator expressions 
as preferred.

Steven D'Aprano

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