[Python-Dev] PEP 8: Discourage named lambdas? (original) (raw)

Mike Klaas mike.klaas at gmail.com
Sat May 3 02:23:27 CEST 2008

On 2-May-08, at 4:03 PM, Terry Reedy wrote:

Some people write somename = lambda args: expression instead of the more obvious (to most people) and, dare I say, standard def somename(args): return expression

The difference in the result (the only one I know of) is that the code and function objects get the generic name '' instead of the more informative (in repr() output or tracebacks) 'somename'. I consider this a disadvantage. In the absence of any compensating advantages (other than the trivial saving of 3 chars), I consider the def form to be the proper Python style to the point I think it should be at least recommended for the stdlib in the Programming Recommendations section of PEP 8. There are currently uses of named lambdas at least in urllib2. This to me is a bad example for new Python programmers. What do our style mavens think?

I'm not a style maven, but I'll put forward why I don't think this is
bad style. Most importantly, these statements can result from
sensible changes from what is (I believe) considered good style.

For example, consider:

registerCallback(lambda: frobnicate(7))

what if there are too places that the callback needs to be registered?

registerCallback(lambda: frobnicate(7)) registerCallback2(lambda: frobnicate(7))

DRY leads to factoring this out into a variable in a straightforward
manner:

callback = lambda: frobnicate(7) registerCallback(callback) registerCallback2(callback)

Another thing to consider is that the def() pattern is only possible
when the bound variable has no dots. A common pattern for me is to
replace an instances method with a lambda to add monitoring hooks or
disable certain functionality:

inst.get_foo = lambda: FakeFoo()

This is not replacable in one line with a def (or without locals()
detritius). Assuming this is good style, it seems odd that

inst.get_foo = lambda: FakeFoo()

is acceptible style, but

get_foo = lambda: FakeFoo()

isn't.

(I also happen to think that the def pattern is less clear in some
situations, but that speaks more to personal taste so isn't
particularly relevant)

-Mike

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