dag.odenhall@gmail.com wrote:

Yes, I know, it's merely a (lambda x: x), but I find a need for this often enough that dedicated, documented function would help readability and encourage certain patterns. Whether it should go in builtins or functools is debatable. A name that is neither in conflict or easily confused with the id() builtin is another problem (the Haskell identity function is called 'id').

So what is the use case? A common example is the pattern of "key functions" as used with sorting: the default is typically the "identity function". Another example is gettext catalogs, which effectively are defaultdicts of the identity function.

I frequently find myself wanting an identify function. Here's a (grossly simplified) example from one of my library functions: def len_sum(iterable, transformation=lambda x:x): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 for x in iterable: count += 1 total += transformation(x) return count, total Except that the overhead of calling the identity function is significant. So I end up repeating myself: def len_sum(iterable, transformation=None): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 if transformation is None: for x in iterable: count += 1 total += x else: ... # you get the picture return count, total So, while I want an identity function, I don't want an identity function which requires actually calling a function at runtime. What I really want is compiler black magic, so that I can write: def len_sum(iterable, transformation=None): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 for x in iterable: count += 1 total += transformation(x) return count, total and the compiler is smart enough to do the Right Thing for me, without either the need to repeat code, or the function call overhead. (And also a pony.) Without that level of black magic, I don't think adding an identity function to the standard library is worth the time or effort. -- Steven

dag.odenhall@... <dag.odenhall@...> writes:

Yes, I know, it's merely a (lambda x: x), but I find a need for this often enough that dedicated, documented function would help readability and encourage certain patterns. Whether it should go in builtins or functools is debatable.

http://mail.python.org/pipermail/python-ideas/2009-March/003646.html

dag.odenhall@gmail.com wrote:

Yes, I know, it's merely a (lambda x: x), but I find a need for this often enough that dedicated, documented function would help readability and encourage certain patterns. Whether it should go in builtins or functools is debatable. A name that is neither in conflict or easily confused with the id() builtin is another problem (the Haskell identity function is called 'id').

So what is the use case? A common example is the pattern of "key functions" as used with sorting: the default is typically the "identity function". Another example is gettext catalogs, which effectively are defaultdicts of the identity function.

I frequently find myself wanting an identify function. Here's a (grossly simplified) example from one of my library functions: def len_sum(iterable, transformation=lambda x:x): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 for x in iterable: count += 1 total += transformation(x) return count, total Except that the overhead of calling the identity function is significant. So I end up repeating myself: def len_sum(iterable, transformation=None): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 if transformation is None: for x in iterable: count += 1 total += x else: ... # you get the picture return count, total So, while I want an identity function, I don't want an identity function which requires actually calling a function at runtime. What I really want is compiler black magic, so that I can write: def len_sum(iterable, transformation=None): """Sum iterable, returning length and sum in one pass.""" count = 0 total = 0 for x in iterable: count += 1 total += transformation(x) return count, total and the compiler is smart enough to do the Right Thing for me, without either the need to repeat code, or the function call overhead. (And also a pony.) Without that level of black magic, I don't think adding an identity function to the standard library is worth the time or effort. -- Steven

dag.odenhall@... <dag.odenhall@...> writes:

Yes, I know, it's merely a (lambda x: x), but I find a need for this often enough that dedicated, documented function would help readability and encourage certain patterns. Whether it should go in builtins or functools is debatable.

http://mail.python.org/pipermail/python-ideas/2009-March/003646.html