Is there a Python equivalent of the Haskell 'let'

Question

Is there a Python equivalent of the Haskell \'let\' expression that would allow me to write something like:

list2 = [let (name,size)=lookup(productId) in (ba         


        

Answer 1

Only guessing at what Haskell does, here's the alternative. It uses what's known in Python as "list comprehension".

[barcode(productId), metric(size)
    for (productId, (name, size)) in [
        (productId, lookup(productId)) for productId in list_]
]

You could include the use of lambda:, as others have suggested.

Answer 2

You could use a temporary list comprehension

[(barcode(productId), metric(size)) for name, size in [lookup(productId)]][0]

or, equivalently, a generator expression

next((barcode(productId), metric(size)) for name, size in [lookup(productId)])

but both of those are pretty horrible.

Another (horrible) method is via a temporary lambda, which you call immediately

(lambda (name, size): (barcode(productId), metric(size)))(lookup(productId))

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I think the recommended "Pythonic" way would just be to define a function, like

def barcode_metric(productId):
   name, size = lookup(productId)
   return barcode(productId), metric(size)
list2 = [barcode_metric(productId) for productId in list]

Answer 3

There is no such thing. You could emulate it the same way let is desugared to lambda calculus (let x = foo in bar <=> (\x -> bar) (foo)).

The most readable alternative depends on the circumstances. For your specific example, I'd choose something like [barcode(productId), metric(size) for productId, (_, size) in zip(productIds, map(lookup, productIds))] (really ugly on second thought, it's easier if you don't need productId too, then you could use map) or an explicit for loop (in a generator):

def barcodes_and_metrics(productIds):
    for productId in productIds:
        _, size = lookup(productId)
        yield barcode(productId), metric(size)