Why are Python lambdas useful? [closed]

Question

I'm trying to figure out Python lambdas. Is lambda one of those "interesting" language items that in real life should be forgotten?

I'm sure there are some edge cases where it might be needed, but given the obscurity of it, the potential of it being redefined in future releases (my assumption based on the various definitions of it) and the reduced coding clarity - should it be avoided?

This reminds me of overflowing (buffer overflow) of C types - pointing to the top variable and overloading to set the other field values. It feels like sort of a techie showmanship but maintenance coder nightmare.

Answer 1

As stated above, the lambda operator in Python defines an anonymous function, and in Python functions are closures. It is important not to confuse the concept of closures with the operator lambda, which is merely syntactic methadone for them.

When I started in Python a few years ago, I used lambdas a lot, thinking they were cool, along with list comprehensions. However, I wrote and have to maintain a big website written in Python, with on the order of several thousand function points. I've learnt from experience that lambdas might be OK to prototype things with, but offer nothing over inline functions (named closures) except for saving a few key-stokes, or sometimes not.

Basically this boils down to several points:

• it is easier to read software that is explicitly written using meaningful names. Anonymous closures by definition cannot have a meaningful name, as they have no name. This brevity seems, for some reason, to also infect lambda parameters, hence we often see examples like lambda x: x+1
• it is easier to reuse named closures, as they can be referred to by name more than once, when there is a name to refer to them by.
• it is easier to debug code that is using named closures instead of lambdas, because the name will appear in tracebacks, and around the error.

That's enough reason to round them up and convert them to named closures. However, I hold two other grudges against anonymous closures.

The first grudge is simply that they are just another unnecessary keyword cluttering up the language.

The second grudge is deeper and on the paradigm level, i.e. I do not like that they promote a functional-programming style, because that style is less flexible than the message passing, object oriented or procedural styles, because the lambda calculus is not Turing-complete (luckily in Python, we can still break out of that restriction even inside a lambda). The reasons I feel lambdas promote this style are:

• There is an implicit return, i.e. they seem like they 'should' be functions.

• They are an alternative state-hiding mechanism to another, more explicit, more readable, more reusable and more general mechanism: methods.

I try hard to write lambda-free Python, and remove lambdas on sight. I think Python would be a slightly better language without lambdas, but that's just my opinion.

Answer 2

Lambda function it's a non-bureaucratic way to create a function.

That's it. For example, let's supose you have your main function and need to square values. Let's see the traditional way and the lambda way to do this:

Traditional way:

def main():
...
...
y = square(some_number)
...
return something

def square(x):
    return x**2

The lambda way:

def main():
...
square = lambda x: x**2
y = square(some_number)
return something

See the difference?

Lambda functions go very well with lists, like lists comprehensions or map. In fact, list comprehension it's a "pythonic" way to express yourself using lambda. Ex:

>>>a = [1,2,3,4]
>>>[x**2 for x in a]
[1,4,9,16]

Let's see what each elements of the syntax means:

[] : "Give me a list"

x**2 : "using this new-born function"

for x in a: "into each element in a"

That's convenient uh? Creating functions like this. Let's rewrite it using lambda:

>>> square = lambda x: x**2
>>> [square(s) for x in a]
[1,4,9,16]

Now let's use map, which is the same thing, but more language-neutral. Maps takes 2 arguments:

(i) one function

(ii) an iterable

And gives you a list where each element it's the function applied to each element of the iterable.

So, using map we would have:

>>> a = [1,2,3,4]
>>> squared_list = map(lambda x: x**2, a)

If you master lambdas and mapping, you will have a great power to manipulate data and in a concise way. Lambda functions are neither obscure nor take away code clarity. Don't confuse something hard with something new. Once you start using them, you will find it very clear.

Answer 3

I use lambda to create callbacks that include parameters. It's cleaner writing a lambda in one line than to write a method to perform the same functionality.

For example:

import imported.module

def func():
    return lambda: imported.module.method("foo", "bar")

as opposed to:

import imported.module

def func():
    def cb():
        return imported.module.method("foo", "bar")
    return cb

Answer 4

I doubt lambda will go away. See Guido's post about finally giving up trying to remove it. Also see an outline of the conflict.

You might check out this post for more of a history about the deal behind Python's functional features: http://python-history.blogspot.com/2009/04/origins-of-pythons-functional-features.html

Curiously, the map, filter, and reduce functions that originally motivated the introduction of lambda and other functional features have to a large extent been superseded by list comprehensions and generator expressions. In fact, the reduce function was removed from list of builtin functions in Python 3.0. (However, it's not necessary to send in complaints about the removal of lambda, map or filter: they are staying. :-)

My own two cents: Rarely is lambda worth it as far as clarity goes. Generally there is a more clear solution that doesn't include lambda.

Answer 5

I find lambda useful for a list of functions that do the same, but for different circumstances. Like the Mozilla plural rules.

plural_rules = [
    lambda n: 'all',
    lambda n: 'singular' if n == 1 else 'plural',
    lambda n: 'singular' if 0 <= n <= 1 else 'plural',
    ...
]
# Call plural rule #1 with argument 4 to find out which sentence form to use.
plural_rule[1](4) # returns 'plural'

If you'd have to define a function for all of those you'd go mad by the end of it. Also it wouldn't be nice with function names like plural_rule_1, plural_rule_2, etc. And you'd need to eval() it when you're depending on a variable function id.

Answer 6

Lambdas are deeply linked to functional programming style in general. The idea that you can solve problems by applying a function to some data, and merging the results, is what google uses to implement most of its algorithms.

Programs written in functional programming style, are easily parallelized and hence are becoming more and more important with modern multi-core machines. So in short, NO you should not forget them.