Don't use lambda for named functions
Some programmers don't understand that in Python, def and lambda define exactly the same kind of function object. (Especially since the equivalent is not true in C++, Ruby, and many other languages.)The differences between def and lambda are:
- def gives you a named function, lambda an anonymous function.
- def lets you include statements in the body, lambda only an expression.
- lambda can be used within an expression, def cannot.
So, when you want to, e.g., define a short callback in the middle of an async request or a GUI widget constructor, especially when it doesn't have an obvious good name, use lamdba.
But when you want to define a named function, use def.
In other words, don't write this:
- iszero = lambda x: hash(x) == hash(0)
(There are other problems with that function, but let's ignore them…)
What's wrong with lambda for named functions?
- Following idioms matters.
- That "iszero" function up there may be bound the the module global name "iszero"—but if it shows up in a traceback, or you print it out at the interactive prompt, or use the inspect module on it, its name is actually <lambda>. That's not nearly as useful.
- The def statement's syntax parallels function call syntax: "def iszero(x):" means it's called with "iszero(x)". That isn't true for "iszero = lambda x:".
- If you later need to change the body to, say, add a try/except around it, you can't do that in a lambda.
But isn't lambda a lot more concise?
In exchange for dropping the 6-letter "return" keyword, and possibly a pair of parens, you've replaced a 3-letter "def" keyword with a 6-letter "lambda" keyword, and added a space and an equals sign. How much more concise do you think that's going to be? Test it yourself; you save 1-3 characters this way; that's it.
If you're thinking that lambda can go on one line and def can't, of course it can:
- def iszero(x): return hash(x) == hash(0)
Sure, PEP 8 says that one-line compound statements are "generally discouraged", but it includes plenty of "Yes" examples that do exactly that; when the recommendations section of a document that itself is only a guideline specifically points out that something is just a "usually don't" rather than a hard "don't", that means something.
And, more importantly, avoiding making your code arguably unpythonic by transforming it into something definitely even more unpythonic is not helping.
And if you're just doing it to trick your linter, I shouldn't have to explain why using a linter but then tricking it is kind of pointless.
Don't use lambda when you don't need a function
In Python, there should be one, and only one, obvious way to do it. And yet we have both comprehensions and higher-order functions like map and filter. Why?
When you have an expression to map or filter with, a comprehension is the obvious way to do it.
When you have a function to map or filter with, the higher-order function is the obvious way to do it.
It's a little silly (and maybe inefficient) to wrap a function call in an expression just to avoid calling map; it's a lot sillier (and more inefficient) to wrap an expression in a function just so you _can_ call map. Compare:
- vals = (x+2*y for x, y in zip(xs, ys)) # good
- vals = map(lambda (x, y): x+2*y, zip(xs, ys)) # bad, and illegal in Python 3
- vals = map(lambda xy: xy[0]+2*xy[1], zip(xs, ys)) # even worse, but legal
The point of your code is not calling a function that adds some numbers, it's just adding some numbers. There's no clearer way to write that than "x+2*y".
Of course the silliest of all is doing both of these things—you have a function, which you wrap in an expression which you then wrap in a function again:
- vals = map(spam, vals) # good
- vals = map(lambda x: spam(x), vals) # really?
Borderline cases
Sometimes, you have a function, but it may not be very obvious to all readers:
- vals = (x+y for x, y in zip(xs, ys)) # good
- vals = map(operator.add, zip(xs, ys)) # also good
Or the only expression that can replace a function is a higher-order expression that some readers may not grasp:
- vals.sort(key=functools.partial(spam, maxval)) # decent
- vals.sort(key=lambda x: spam(maxval, x)) # also decent
Here, it's really a judgment call. It depends who you expect to be reading your code, and what feels most natural to you, and maybe even what the surrounding code looks like.
Of course when you need to do something that you can't quite do with the higher-order tools that Python has provided, it's even more obviously a judgment call. You don't want to write (or install off PyPI) a right-partial, argument-flip, or compose function just to avoid one use of lambda—but to avoid 200 uses of lambda throughout a project, maybe you do.
Comprehensions only handle map and filter
As great as comprehensions are, they don't let you replace functools.reduce or itertools.dropwhile, only map and filter. If you are going to use functions like that, you don't have any choice but to wrap up your transforming expression or predicate test in a function. Of course sometimes that's a reason not to use those functions (Guido likes to say inside every reduce is a for loop screaming to get out), but sometimes it isn't. Again, each case is a judgment call.
So why do we even have lambda?
Because every time Guido suggests getting rid of it, the rest of the community shouts him down. :)
More seriously, lambda is great for exactly the kinds of examples I gave above—a short and simple button callback, or sorting key, or dropwhile predicate; doing something that's just outside the grasp of partial; etc.
Sometimes you need a function that's anonymous, trivial, and can be written in-line within an expression. And that's why we have lambda.
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