Re: Mapping None. Why?
On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy
[EMAIL PROTECTED] wrote:
Iam wondering why the peculiar behavior of map when the function in
given as None:
If you start with a value x and then apply no function
at all to it, what results is x.
Help on built-in function map in module __builtin__:
map(...)
map(function, sequence[, sequence, ...]) - list
Return a list of the results of applying the function to the items
of
the argument sequence(s). If more than one sequence is given, the
function is called with an argument list consisting of the
corresponding
item of each sequence, substituting None for missing values when
not all
sequences have the same length. If the function is None, return a
list of
the items of the sequence (or a list of tuples if more than one
sequence).
It seems as the action whith none is the same as using a function of
lambda *x: x
As in the following example:
l1 = 'asdf'
l2 = 'qwertyuip'
l3 = range(3)
l1,l2,l3
('asdf', 'qwertyuip', [0, 1, 2])
map(lambda *x: x, l1,l2,l3) == map(None, l1,l2,l3)
True
On looking up map on Wikipedia there is no mention of this special
behaviour,
So my question is why?
Thanks, Paddy.
David C. Ullrich
--
http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Jun 13, 12:49 pm, David C. Ullrich [EMAIL PROTECTED] wrote: On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy [EMAIL PROTECTED] wrote: Iam wondering why the peculiar behavior of map when the function in given as None: If you start with a value x and then apply no function at all to it, what results is x. David C. Ullrich True, but None is not a function. It's a sentinel value to turn on the functionality. - Paddy. -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
In article [EMAIL PROTECTED], Paddy [EMAIL PROTECTED] wrote: On Jun 13, 12:49 pm, David C. Ullrich [EMAIL PROTECTED] wrote: On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy [EMAIL PROTECTED] wrote: Iam wondering why the peculiar behavior of map when the function in given as None: If you start with a value x and then apply no function at all to it, what results is x. David C. Ullrich True, but None is not a function. It's a sentinel value to turn on the functionality. Uh, thanks. I think I knew that - I was just suggesting why the way map works makes sense. - Paddy. -- David C. Ullrich -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
David C. Ullrich [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] | In article | [EMAIL PROTECTED], | Paddy [EMAIL PROTECTED] wrote: | | True, but None is not a function. It's a sentinel value to turn on the | functionality. | | Uh, thanks. I think I knew that - I was just suggesting why | the way map works makes sense. filter(None, iterable) works the same way: None- identity function, The immediate reason is the Python has no builtin id(). But apparently there is also historical precedent in the functional community for this convention. -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Fri, Jun 13, 2008 at 2:00 PM, Terry Reedy [EMAIL PROTECTED] wrote: filter(None, iterable) works the same way: None- identity function, The immediate reason is the Python has no builtin id(). But apparently there is also historical precedent in the functional community for this convention. Another way of viewing it is that filter(None, iterable) applies no function at all before testing the truth values, which does make some sense. With map, however, this is not strictly true. -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
Paddy schrieb:
Iam wondering why the peculiar behavior of map when the function in
given as None:
Help on built-in function map in module __builtin__:
map(...)
map(function, sequence[, sequence, ...]) - list
Return a list of the results of applying the function to the items
of
the argument sequence(s). If more than one sequence is given, the
function is called with an argument list consisting of the
corresponding
item of each sequence, substituting None for missing values when
not all
sequences have the same length. If the function is None, return a
list of
the items of the sequence (or a list of tuples if more than one
sequence).
It seems as the action whith none is the same as using a function of
lambda *x: x
As in the following example:
l1 = 'asdf'
l2 = 'qwertyuip'
l3 = range(3)
l1,l2,l3
('asdf', 'qwertyuip', [0, 1, 2])
map(lambda *x: x, l1,l2,l3) == map(None, l1,l2,l3)
True
On looking up map on Wikipedia there is no mention of this special
behaviour,
So my question is why?
Because it is undefined what should happen in case of no function given
at all - and because there is no identity function in python
pre-defined, it could be considered sensible to make None the quivalent
of that function.
And it only follows that *if* you imply a function even though there is
None given, that the passed tuple is returned.
I don't see anything on wikipedia that defines any other behavior.
Diez
Diez
--
http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Thu, Jun 12, 2008 at 1:05 PM, Paddy [EMAIL PROTECTED] wrote: Iam wondering why the peculiar behavior of map when the function in given as None: Because that's the way it's always been! Seriously, I don't know. I can tell you that it's going away in Python 3.0, though. Ian -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Thu, Jun 12, 2008 at 1:32 PM, Diez B. Roggisch [EMAIL PROTECTED] wrote: Because it is undefined what should happen in case of no function given at all - and because there is no identity function in python pre-defined, it could be considered sensible to make None the quivalent of that function. It makes more sense to raise an error when a non-function is passed where a function is expected. If we're going to have a special behaviour for None, why not have special behaviours for True, False, and 42 as well? The proper solution to the lack of a built-in packing (not identity) function is to define a packing function, not to special-case an arbitrary value to *mean* the packing function in certain situations. Ian -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
Ian Kelly wrote: On Thu, Jun 12, 2008 at 1:05 PM, Paddy [EMAIL PROTECTED] wrote: Iam wondering why the peculiar behavior of map when the function in given as None: Because that's the way it's always been! Seriously, I don't know. I can tell you that it's going away in Python 3.0, though. There was a time before zip(). Basically, it's a really useful feature, and the original implementor thought that map() was a reasonable place to put it; it's a somewhat natural outgrowth of the map(func, list1, list2, ... listn) semantics. Since then, after more thought was put into it, it was realized that a separate builtin function is more appropriate for this common use, and thus zip() was born and map(None, ...) was deprecated. -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
Ian Kelly schrieb: On Thu, Jun 12, 2008 at 1:32 PM, Diez B. Roggisch [EMAIL PROTECTED] wrote: Because it is undefined what should happen in case of no function given at all - and because there is no identity function in python pre-defined, it could be considered sensible to make None the quivalent of that function. It makes more sense to raise an error when a non-function is passed where a function is expected. If we're going to have a special behaviour for None, why not have special behaviours for True, False, and 42 as well? The proper solution to the lack of a built-in packing (not identity) function is to define a packing function, not to special-case an arbitrary value to *mean* the packing function in certain situations. You are right with the packing-function, it's not identity of course. However I don't see that this as an area that is really important (especially since map is being replaced by list-comps most of the time). And the OP's question was about map not being conforming to the definition on wikipedia - which I don't think it's not. It is not defined what map is to do with None (or NULL or nil or... ) as argument. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
Paddy [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] | | Iam wondering why the peculiar behavior of map when the function in | given as None: The 'peculiar behavior' is the same as zip (except for padding short iterators versus truncating long iterators. Map was added years before zip. After that, map(None,...) was kept for back compatibility. In 3.0, the doc for map is Return an iterator that applies function to every item of iterable, yielding the results. If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. With multiple iterables, the iterator stops when the shortest iterable is exhausted. Using a map defined with None raises TypeError: 'NoneType' object is not callable tjr -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
Paddy wrote: On looking up map on Wikipedia there is no mention of this special behaviour, So my question is why? My question is why you are looking up the semantics of Python functions on Wikipedia instead of the Python documentation. I don't see any particular discussion of map() there at all. Am I missing something? -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Jun 12, 8:55 pm, Diez B. Roggisch [EMAIL PROTECTED] wrote: And the OP's question was about map not being conforming to the definition on wikipedia - which I don't think it's not. It is not defined what map is to do with None (or NULL or nil or... ) as argument. Diez Oh no! Sorry to give that impression. I don't think that map should be like what Wikipedia says, I was just looking for another example of an implementation that might mention the behaviour. I just want to know the thoughts behind this behaviour in the Python map. - Paddy. -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Jun 12, 9:48 pm, Robert Kern [EMAIL PROTECTED] wrote: Paddy wrote: On looking up map on Wikipedia there is no mention of this special behaviour, So my question is why? My question is why you are looking up the semantics of Python functions on Wikipedia instead of the Python documentation. I don't see any particular discussion of map() there at all. Am I missing something? -- Robert Kern As I said in an answer to Diez B. Roggish, I had been reminded of the behaviour, thought it odd, and looked for other implementations that might have the behaviour via wikipedia. My intension was most definitely NOT to say that Pythons map should do what Wikipedia says slavishly. Sometimes when I pick at these seeming inconsistencies I learn a lot from the c.l.p replies. Someone elses thread on -0.0 versus +0.0 taught me some more on floating point for example. - Paddy. -- http://mail.python.org/mailman/listinfo/python-list
Re: Mapping None. Why?
On Jun 12, 9:36 pm, Terry Reedy [EMAIL PROTECTED] wrote: Paddy [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] | | Iam wondering why the peculiar behavior of map when the function in | given as None: The 'peculiar behavior' is the same as zip (except for padding short iterators versus truncating long iterators. Map was added years before zip. After that, map(None,...) was kept for back compatibility. In 3.0, the doc for map is Return an iterator that applies function to every item of iterable, yielding the results. If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. With multiple iterables, the iterator stops when the shortest iterable is exhausted. Using a map defined with None raises TypeError: 'NoneType' object is not callable tjr I really should get into the habit of reading the 3.0 docs before asking questions :-) My original question came about after answering this query: http://gmcnaughton.livejournal.com/27955.html?thread=70451#t70451 - Paddy. -- http://mail.python.org/mailman/listinfo/python-list
