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

I started reading David Mertz's book today 'Text Processing in Python.' While he has a fairly terse description of Lambda's the examples in the first chapter combined with the explanation in Appendix A made them jump off the page for me (finally) and all of a sudden I understood their value. That is not to say his explanation will work for you and I am still at the discovery stage so I will not attempt to add to these responses other than the following: I am new to Python I am new to OOP Lambdas were a struggle for me Now that I read Mertz, I think I get them and I see them as very useful as I think they allow a cleaner approach to programming.

He reproduces the Zen of Python, one line of which is Simple is better than complex. As a non-OOP programmer reading code with lambdas (and until last week list comprehensions) I have thought-This is simple?. I finally realized today that actually these features make the code much more readable, and understandable than the alternative-which is invariably a loop of some sort. I also realized that like financial statements-Python was not designed for the novice user, rather it is designed for the user that wants to get educated. I can't believe how powerful this language is. When it dawned on me (finally) the purpose and value of lambdas I wanted to rip up about 30 programs and start over putting in lambdas where appropriate.

Answer 2

Are you talking about lambda functions? Like

lambda x: x**2 + 2*x - 5

Those things are actually quite useful. Python supports a style of programming called functional programming where you can pass functions to other functions to do stuff. Example:

mult3 = filter(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9])

sets mult3 to [3, 6, 9], those elements of the original list that are multiples of 3. This is shorter (and, one could argue, clearer) than

def filterfunc(x):
    return x % 3 == 0
mult3 = filter(filterfunc, [1, 2, 3, 4, 5, 6, 7, 8, 9])

Of course, in this particular case, you could do the same thing as a list comprehension:

mult3 = [x for x in [1, 2, 3, 4, 5, 6, 7, 8, 9] if x % 3 == 0]

(or even as range(3,10,3)), but there are many other, more sophisticated use cases where you can't use a list comprehension and a lambda function may be the shortest way to write something out.

• Returning a function from another function

  >>> def transform(n):
  ...     return lambda x: x + n
  ...
  >>> f = transform(3)
  >>> f(4)
  7
  

  This is often used to create function wrappers, such as Python's decorators.

• Combining elements of an iterable sequence with reduce()

  >>> reduce(lambda a, b: '{}, {}'.format(a, b), [1, 2, 3, 4, 5, 6, 7, 8, 9])
  '1, 2, 3, 4, 5, 6, 7, 8, 9'
  

• Sorting by an alternate key

  >>> sorted([1, 2, 3, 4, 5, 6, 7, 8, 9], key=lambda x: abs(5-x))
  [5, 4, 6, 3, 7, 2, 8, 1, 9]
  

I use lambda functions on a regular basis. It took me a while to get used to them, but eventually I came to understand that they're a very valuable part of the language.

Answer 3

The two-line summary:

1. Closures: Very useful. Learn them, use them, love them.
2. Python's lambda keyword: unnecessary, occasionally useful. If you find yourself doing anything remotely complex with it, put it away and define a real function.

Answer 4

I've been using Python for a few years and I've never run in to a case where I've needed lambda. Really, as the tutorial states, it's just for syntactic sugar.

Answer 5

I can give you an example where I actually needed lambda serious. I'm making a graphical program, where the use right clicks on a file and assigns it one of three options. It turns out that in Tkinter (the GUI interfacing program I'm writing this in), when someone presses a button, it can't be assigned to a command that takes in arguments. So if I chose one of the options and wanted the result of my choice to be:

print 'hi there'

Then no big deal. But what if I need my choice to have a particular detail. For example, if I choose choice A, it calls a function that takes in some argument that is dependent on the choice A, B or C, TKinter could not support this. Lamda was the only option to get around this actually...

Answer 6

A lambda is part of a very important abstraction mechanism which deals with higher order functions. To get proper understanding of its value, please watch high quality lessons from Abelson and Sussman, and read the book SICP

These are relevant issues in modern software business, and becoming ever more popular.