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 (barcode(productId),metric(size)) 
            for productId in list]

If not, what would be the most readable alternative?

Added for clarification of the let syntax:

x = let (name,size)=lookup(productId) in (barcode(productId),metric(size))

is equivalent to

(name,size) = lookup(productId)
x = (barcode(productId),metric(size))

The second version doesn’t work that well with list comprehensions, though.

Answer 1

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]