5 ms·

> multi-line anonymous functions This is maybe a bit far from the topic, but I would like to know why there is such a need for anonymous functions in Python.

by gbog 15y ago

> multi-line anonymous functions This is maybe a bit far from the topic, but I would like to know why there is such a need for anonymous functions in Python. I understand very well the need for stateless functions (functions that do not change anything else than their return value, have no side effect, except maybe logging), and I understood it was a piece of functional programming. In Javascript, with silent overwriting of functions and without modules, it is obviously very bad to pollute the only namespace with function names that can clash with each other, but in Python, you are or should be always inside a relatively small scope, and there you can def _my_temp_func() without any problems, no? Moreover, as it is lame to use _temp as function name, it forces you to formulate a meaningful function name, which is good pratice, especially for your lectorate, right? Or am I missing something?

burgerbrain 15y ago
I don't know python, so I don't know if anonymous functions have been crippled there, but in other languages anonymous functions are used to do things such as create closures (not stateless). For example, in scheme: > (define (foo x) (lambda (y) (+ x y))) > (define bar (foo 10)) > (bar 5) 15 This sort of thing is of course immensely useful.
- andrewaylett 15y ago
  You can do the same in Python, with named functions or lambdas: >>> def foo(x): ... def add(y): ... return x + y ... return add ... >>> bar = foo(10) >>> bar(5) 15
  - masklinn 15y ago
    And it takes twice the token count and lowered readability for no gain whatsoever.
    - burgerbrain 15y ago
      No gain at all? How do you figure?
    - olavk 15y ago
      Twice the token count? How did you get that?
    - cyrus_ 15y ago
      foo = lambda x: lambda y: x + y bar = foo(10) This is perfectly correct Python code too if you prefer. The only real thing with Python's lambdas is that, because Python makes a distinction between statements and expressions, you can't easily do things like use temporary variables inside a lambda, but cases like that are rare and are typically more readable with a separate standalone function definition anyway.
tsuraan 15y ago
lambdas can be really nice for use with python's more functional builtins, like filter, map, and sort. As an example, suppose you have a list like this: x = [{1: 'b', 2: 'a'}, {1:'a', 2:'b'}] You want to sort it on the value at the "2" key. You could write a dummy function: def extract_2(dct): return dct[2] x.sort(key=extract_2) Or, you could just use a quick lambda: x.sort(key=lambda dct: dct[2]) The latter is more terse, and to a functional programmer, more clear (YMMV, of course). Lambda is just syntactic sugar in python; it's not necessary, and a lot of places where a functional programmer uses it in python make more sense as list comprehensions, but I think it's a nice thing to have. EDIT: I think the biggest reason I prefer the lambda approach to the named-function approach is that I don't like seeing one-off functions defined first and then seeing them used later. If python could do this: x.sort(key=extract_2) def extract_2(dct): return dct[2] or even somehow support ruby-style blocks, it wouldn't be so ugly to my eye. Python lets functions call functions that are defined below them at the module level, but not at the function level, so the above isn't valid in the scope of a function.
masklinn 15y ago
> This is maybe a bit far from the topic, but I would like to know why there is such a need for anonymous functions in Python. Building new control structures in-language, without having to hack the interpreter, and without the result looking like somebody has puked all over your screen. First-class functions are under-used in Python because they're syntactically and semantically heavy. With full anonymous functions, the `with` statement would not have been needed, it could have been handled through a higher-order function, akin to Haskell's `bracket`[0] > In Javascript, with silent overwriting of functions and without modules, it is obviously very bad to pollute the only namespace with function names that can clash with each other Namespace pollution is completely irrelevant (it's very easy to handle it in Javascript via the namespace pattern). > Moreover, as it is lame to use _temp as function name, it forces you to formulate a meaningful function name, which is good pratice, especially for your lectorate, right? If it is, why can I put name-less blocks of code in `for`, `if`, `while` and `with` bodies? Not all code blocks need a name. Those that do will be given one. [0] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception-Base.html#v:bracket http://hackage.haskell.org/packages/archive/base/latest/doc/...
- thurn 15y ago
  In fact, Haskell style frowns on using too many lambdas. It's generally felt that adding named functions is better for debugging and readability, the same as in python. So with the bracket function, for example, it's still considered better practice to pass named functions to it.
  - masklinn 15y ago
    > In fact, Haskell style frowns on using too many lambdas. It's generally felt that adding named functions is better for debugging and readability, the same as in python That's really besides my point. And last time I checked, Haskell's `if` and `case` expressions did not take functions, they took anonymous expressions. > So with the bracket function, for example, it's still considered better practice to pass named functions to it. To pass a named function as third argument to bracket?
- deleted 15y ago

by gbog 15y ago

burgerbrain 15y ago

I don't know python, so I don't know if anonymous functions have been crippled there, but in other languages anonymous functions are used to do things such as create closures (not stateless). For example, in scheme: > (define (foo x) (lambda (y) (+ x y))) > (define bar (foo 10)) > (bar 5) 15 This sort of thing is of course immensely useful.

andrewaylett 15y ago
You can do the same in Python, with named functions or lambdas: >>> def foo(x): ... def add(y): ... return x + y ... return add ... >>> bar = foo(10) >>> bar(5) 15
- masklinn 15y ago
  And it takes twice the token count and lowered readability for no gain whatsoever.
  - burgerbrain 15y ago
    No gain at all? How do you figure?
  - olavk 15y ago
    Twice the token count? How did you get that?
  - cyrus_ 15y ago
    foo = lambda x: lambda y: x + y bar = foo(10) This is perfectly correct Python code too if you prefer. The only real thing with Python's lambdas is that, because Python makes a distinction between statements and expressions, you can't easily do things like use temporary variables inside a lambda, but cases like that are rare and are typically more readable with a separate standalone function definition anyway.

tsuraan 15y ago

lambdas can be really nice for use with python's more functional builtins, like filter, map, and sort. As an example, suppose you have a list like this: x = [{1: 'b', 2: 'a'}, {1:'a', 2:'b'}] You want to sort it on the value at the "2" key. You could write a dummy function: def extract_2(dct): return dct[2] x.sort(key=extract_2) Or, you could just use a quick lambda: x.sort(key=lambda dct: dct[2]) The latter is more terse, and to a functional programmer, more clear (YMMV, of course). Lambda is just syntactic sugar in python; it's not necessary, and a lot of places where a functional programmer uses it in python make more sense as list comprehensions, but I think it's a nice thing to have. EDIT: I think the biggest reason I prefer the lambda approach to the named-function approach is that I don't like seeing one-off functions defined first and then seeing them used later. If python could do this: x.sort(key=extract_2) def extract_2(dct): return dct[2] or even somehow support ruby-style blocks, it wouldn't be so ugly to my eye. Python lets functions call functions that are defined below them at the module level, but not at the function level, so the above isn't valid in the scope of a function.

masklinn 15y ago

> This is maybe a bit far from the topic, but I would like to know why there is such a need for anonymous functions in Python. Building new control structures in-language, without having to hack the interpreter, and without the result looking like somebody has puked all over your screen. First-class functions are under-used in Python because they're syntactically and semantically heavy. With full anonymous functions, the `with` statement would not have been needed, it could have been handled through a higher-order function, akin to Haskell's `bracket`[0] > In Javascript, with silent overwriting of functions and without modules, it is obviously very bad to pollute the only namespace with function names that can clash with each other Namespace pollution is completely irrelevant (it's very easy to handle it in Javascript via the namespace pattern). > Moreover, as it is lame to use _temp as function name, it forces you to formulate a meaningful function name, which is good pratice, especially for your lectorate, right? If it is, why can I put name-less blocks of code in `for`, `if`, `while` and `with` bodies? Not all code blocks need a name. Those that do will be given one. [0] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception-Base.html#v:bracket http://hackage.haskell.org/packages/archive/base/latest/doc/...

thurn 15y ago
In fact, Haskell style frowns on using too many lambdas. It's generally felt that adding named functions is better for debugging and readability, the same as in python. So with the bracket function, for example, it's still considered better practice to pass named functions to it.
- masklinn 15y ago
  > In fact, Haskell style frowns on using too many lambdas. It's generally felt that adding named functions is better for debugging and readability, the same as in python That's really besides my point. And last time I checked, Haskell's `if` and `case` expressions did not take functions, they took anonymous expressions. > So with the bracket function, for example, it's still considered better practice to pass named functions to it. To pass a named function as third argument to bracket?
deleted 15y ago