Re: How to create functors?
Steven D'Aprano ste...@remove.this.cybersource.com.au writes:
According to Wikipedia, functor can be used as a synonym for function
object: ... http://en.wikipedia.org/wiki/Function_object
Hmm, I hadn't seen that usage before. I guess there's no law against
it, but it seems a bit bogus to me.
I find the Haskell page entirely opaque and unintelligible... The
Wikipedia page is a little better
I thought it was the other way around, but I've been banging my head
against Haskell on and off for the past year or so, so it could be
that the Haskell page is more intelligible if the reader already has
some familiarity with Haskell and its type system.
Here's another attempt of my own, from the mathematical perspective:
categories generalize the concept of structured collections of
objects. For example, groups, rings, and sets are each a type of
structured collection, so there is a category of groups (called Grp
since for some reason categorists like to use capitalized, abbreviated
names), a category of rings (Rng), and a category of sets (Set).
Each category has a collection of objects and a collection of arrows
(sometimes called morphisms) which are associative relations between
objects in the category. So in the category of sets, the objects are
sets and the arrows are functions mapping one set to another. In the
category of groups, the objects are groups and the arrows are the
homeomorphisms between groups.
Functors are mappings from one category to another, that map both the
objects and the arrows. That is they are arrows on the categories of
categories.
The concepts of categories and functors came from the loftier reaches
of algebraic geometry (further in math than I ever got) in the 1950's
or so. These days they turn out to be useful in studying type systems
of programming languages, which is where most of my exposure to them
has come from.
But let me try an example to see if I've got it right:
class Int2StrFunctor:
def map1(self, n):
if type(n) is not int:
raise TypeError('argument must be an int')
I don't think this is right. A functor (in the PL sense that I've
been describing) acts on types, not on members of types. That is,
your example turns 3 into ---, i.e. it turns a member of type
int into a member of type str. A functor would be something that
acts on arbitrary types, e.g. one might turn int into array of
int, and str into array of str and so forth. Another might turn
int into red-black tree containing a pair of ints at each node and
doing something similar with str. The map1 and map2 functions (I
guess it is ok to call them that) are supposed to be generic.
I guess you could have a String functor whose map1 function is repr
and whose map2 functor is (lambda f: lambda x,f=f: repr(f(x))) or
something like that. It would transport every type to the str type.
There are some technical restrictions on functors, relating to the sorts
of functions and types (strictly categories) they can accept,
presumably to make them mathematically well-behaved.
Right.
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Re: How to create functors?
As near as I can tell, a functor is just an object which is callable like a function I believe that's how they're defined in the C++ world, in which, of course, functions aren't first-class objects... - Rami Chowdhury Never assume malice when stupidity will suffice. -- Hanlon's Razor 408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD) On Aug 19, 2009, at 21:11 , Steven D'Aprano wrote: On Wed, 19 Aug 2009 18:42:32 -0700, Paul Rubin wrote: Robert Dailey rcdai...@gmail.com writes: I want to simply wrap a function up into an object so it can be called with no parameters. Nitpick: what you are asking for is called a closure. Functor means something completely different. I'm glad somebody else noticed this. I would have said something about it myself, except I wasn't entirely sure my understanding of functor is correct. As near as I can tell, a functor is just an object which is callable like a function without actually being implemented as a function, e.g.: class Functor: def __call__(self): return None f = Functor() result = f() -- Steven -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
Steven D'Aprano ste...@remove.this.cybersource.com.au writes: As near as I can tell, a functor is just an object which is callable like a function without actually being implemented as a function, e.g.: No it's not anything like that either, at least as I'm used to the term in programming or in mathematics. Maybe it's used other ways though. As I'm used to it, it's a feature of certain static type systems. The notion isn't that useful in Python but you could translate it something like this: imagine that the list datatype has a couple extra operations: # lift_value turns a normal value from a base type to a 1-element list list.lift_value(x) = [x] # lift_function turns a function on a base type to a higher order # function that operates on a whole list list.lift_function(f) = partial(map, f) Then given a function like def square(x): return x*x you could say lifted_square = list.lifted_function(square) print lifted_square([1,2,3,4,5]) and get [1,4,9,16,25]. Similarly if you had some other type (like a tree data structure), that type could also support a map-like operation so you could lift functions to it (lifting a function and applying it to a tree like [1,[2,3],4] would result in [1,[4,9],16] or whatever). If I remember properly, for type t to be a functor it needs the above two lifting operations, with the property that for a given function f, t.lifted_value(f(x)) = (t.lifted_function(f))(t.lifted_value(x)) I guess this explanation isn't going that well but basically in a statically typed language you'd use functors to convert the signatures of functions to versions that operate on higher types. The idea comes from ML and is used explicitly in Haskell, and I think some of the C++ standard template library can be explained in terms of functors. For the mathematical concept of functors and how they relate to programming (at least Haskell's version), see: http://en.wikibooks.org/Haskell/Category_theory -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Thu, 20 Aug 2009 01:36:14 -0700, Paul Rubin wrote:
Steven D'Aprano ste...@remove.this.cybersource.com.au writes:
As near as I can tell, a functor is just an object which is callable
like a function without actually being implemented as a function, e.g.:
No it's not anything like that either, at least as I'm used to the term
in programming or in mathematics. Maybe it's used other ways though.
According to Wikipedia, functor can be used as a synonym for function
object:
http://en.wikipedia.org/wiki/Function_object
which is what I was thinking of. So it seems there are at least two
meanings for the word, neither of which seems to apply to this thread :)
As I'm used to it, it's a feature of certain static type systems. The
notion isn't that useful in Python
I find the Haskell page entirely opaque and unintelligible. Well, perhaps
not *entirely* opaque, but pretty close: it assumes a mathematical
sophistication that I don't think I even had when I was getting my maths
degree, let alone can remember two decades later. (Pity the poor VB
coders wondering what Haskell is good for...) The Wikipedia page is a
little better, but it's section on Examples is laughable -- the examples
are as unintelligible to this reader as the description before them.
But let me try an example to see if I've got it right:
class Int2StrFunctor:
def map1(self, n):
if type(n) is not int:
raise TypeError('argument must be an int')
return -*n
def map2(self, f):
if type(f) is not type(lambda: None):
raise TypeError('argument must be a function')
# assume f takes an int, and returns another int
def inner(n):
return self.map1(f(n))
return inner
The functor can take an int and return a string:
F = Int2StrFunctor() # F is a functor
F.map1(3)
'---'
It can also take a function (of int - int) and return a new function
(int - str):
def myfunc(n):
... return n+2
...
f = F.map2(myfunc)
f(3)
'-'
f(4)
'--'
There's nothing special about the methods map1() and map2(), I could call
them anything I like, or even do this:
def __call__(self, arg):
... if type(arg) is int:
... return self.map1(arg)
... else:
... return self.map2(arg)
...
Int2StrFunctor.__call__ = __call__
F(2)
'--'
F(myfunc)(0)
'--'
There are some technical restrictions on functors, relating to the sorts
of functions and types (strictly categories) they can accept,
presumably to make them mathematically well-behaved.
Have I got it correct?
--
Steven
--
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Re: How to create functors?
On Aug 20, 2009, at 5:25 AM, Steven D'Aprano wrote:
On Thu, 20 Aug 2009 01:36:14 -0700, Paul Rubin wrote:
Steven D'Aprano ste...@remove.this.cybersource.com.au writes:
As near as I can tell, a functor is just an object which is callable
like a function without actually being implemented as a function,
e.g.:
No it's not anything like that either, at least as I'm used to the
term
in programming or in mathematics. Maybe it's used other ways though.
According to Wikipedia, functor can be used as a synonym for function
object:
http://en.wikipedia.org/wiki/Function_object
which is what I was thinking of. So it seems there are at least two
meanings for the word, neither of which seems to apply to this
thread :)
As I'm used to it, it's a feature of certain static type systems.
The
notion isn't that useful in Python
I find the Haskell page entirely opaque and unintelligible. Well,
perhaps
not *entirely* opaque, but pretty close: it assumes a mathematical
sophistication that I don't think I even had when I was getting my
maths
degree, let alone can remember two decades later. (Pity the poor VB
coders wondering what Haskell is good for...) The Wikipedia page is a
little better, but it's section on Examples is laughable -- the
examples
are as unintelligible to this reader as the description before them.
To this reader -- an Rb coder -- the examples were pretty clear.
But let me try an example to see if I've got it right:
class Int2StrFunctor:
def map1(self, n):
if type(n) is not int:
raise TypeError('argument must be an int')
return -*n
def map2(self, f):
if type(f) is not type(lambda: None):
raise TypeError('argument must be a function')
# assume f takes an int, and returns another int
def inner(n):
return self.map1(f(n))
return inner
The functor can take an int and return a string:
F = Int2StrFunctor() # F is a functor
F.map1(3)
'---'
It can also take a function (of int - int) and return a new function
(int - str):
def myfunc(n):
... return n+2
...
f = F.map2(myfunc)
f(3)
'-'
f(4)
'--'
There's nothing special about the methods map1() and map2(), I could
call
them anything I like, or even do this:
def __call__(self, arg):
... if type(arg) is int:
... return self.map1(arg)
... else:
... return self.map2(arg)
...
Int2StrFunctor.__call__ = __call__
F(2)
'--'
F(myfunc)(0)
'--'
There are some technical restrictions on functors, relating to the
sorts
of functions and types (strictly categories) they can accept,
presumably to make them mathematically well-behaved.
Have I got it correct?
I don't think so. Paul Rubin's square example was, I thought,
particularly instructive.
Charles Yeomans--
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Re: How to create functors?
Terry Reedy a écrit :
Robert Dailey wrote:
I'm using Python 2.6. And using the legacy syntax in the lambda does
not work either. I want to avoid using a def if possible. Thanks.
In Python, writing
name = lambda arg: expr
instead of
def name(arg): return expr
is all negative and no positive and should be avoided.
Except that def is a statement, and as such can't be used as a named
params when calling a function expecting a callback, ie:
vroom = some_dead('parrot', name=lambda arg: exp)
(notice the 'name = lambda arg: exp' ?-)
Ok, nitpicking. Me ---[]
g
--
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Re: How to create functors?
On Wed, 2009-08-19 at 15:56 +0200, Bruno Desthuilliers wrote:
Terry Reedy a écrit :
Robert Dailey wrote:
I'm using Python 2.6. And using the legacy syntax in the lambda does
not work either. I want to avoid using a def if possible. Thanks.
In Python, writing
name = lambda arg: expr
instead of
def name(arg): return expr
is all negative and no positive and should be avoided.
Except that def is a statement, and as such can't be used as a named
params when calling a function expecting a callback, ie:
vroom = some_dead('parrot', name=lambda arg: exp)
(notice the 'name = lambda arg: exp' ?-)
Which offers no added functionality over:
def name_func(arg):
return exp
vroom = some_dead('parrot', name=name_func)
except for confusion in debugging. :) (See other excessively long
threads on lambda for further discussion of the debugging headaches
caused by lambdas.
Ok, nitpicking. Me ---[]
g
--
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Re: How to create functors?
Robert Dailey rcdai...@gmail.com writes: I want to simply wrap a function up into an object so it can be called with no parameters. Nitpick: what you are asking for is called a closure. Functor means something completely different. As a few other people have explained, print in python 2.x is a statement rather than a function. You could use sys.stdout.write instead of print. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Wed, 19 Aug 2009 18:42:32 -0700, Paul Rubin wrote: Robert Dailey rcdai...@gmail.com writes: I want to simply wrap a function up into an object so it can be called with no parameters. Nitpick: what you are asking for is called a closure. Functor means something completely different. I'm glad somebody else noticed this. I would have said something about it myself, except I wasn't entirely sure my understanding of functor is correct. As near as I can tell, a functor is just an object which is callable like a function without actually being implemented as a function, e.g.: class Functor: def __call__(self): return None f = Functor() result = f() -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
Robert Dailey wrote: I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. In Python, writing name = lambda arg: expr instead of def name(arg): return expr is all negative and no positive and should be avoided. Your experience illustrates one reason why. tjr -- http://mail.python.org/mailman/listinfo/python-list
How to create functors?
Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. Seems like it works fine on everything else except for print(). For example: print1 = lambda: MyFunction( FooBar ) The syntax above is accepted by the interpreter. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Tue, Aug 18, 2009 at 1:32 PM, Robert Daileyrcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. ch...@morpheus ~ $ python Python 2.6.2 (r262:71600, May 14 2009, 16:34:51) [GCC 4.0.1 (Apple Inc. build 5484)] on darwin Type help, copyright, credits or license for more information. print1 = lambda: print( Foobar ) File stdin, line 1 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax from __future__ import print_function print1 = lambda: print( Foobar ) print1() Foobar Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Aug 18, 3:40 pm, Chris Rebert c...@rebertia.com wrote: On Tue, Aug 18, 2009 at 1:32 PM, Robert Daileyrcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. ch...@morpheus ~ $ python Python 2.6.2 (r262:71600, May 14 2009, 16:34:51) [GCC 4.0.1 (Apple Inc. build 5484)] on darwin Type help, copyright, credits or license for more information. print1 = lambda: print( Foobar ) File stdin, line 1 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax from __future__ import print_function print1 = lambda: print( Foobar ) print1() Foobar Cheers, Chris --http://blog.rebertia.com I see what you're saying now. However, why am I able to use print as a function in general-purpose code in my Python 2.6 script, like so: def SomeFunction(): print( Hello World ) But, I am not able to do this: SomeFunction = lambda: print( Hello World ) ?? Doesn't make sense. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
Lambda expressions are, I believe, syntactically limited to a single expression -- no statements, like 'print' is in Python 2.x. If you are strongly against just defining a function, you might have to use a trick to get around it -- this page (http://p-nand-q.com/python/stupid_lambda_tricks.html) has some suggestions. On Tue, 18 Aug 2009 13:32:55 -0700, Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. -- Rami Chowdhury Never attribute to malice that which can be attributed to stupidity -- Hanlon's Razor 408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD) -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
In article c217c4a4-b891-469e-af4f-4e44e2c95...@z24g2000yqb.googlegroups.com, Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. The problem is that in Python 2 print is a statement, not a function. That should work fine in Python 3 where print *is* a function. In 2.x, you can wrap print in a function or use something like: import sys print1 = lambda: sys.stdout.write(Foobar\n) print1() Foobar or the pprint library module or various other solutions. -- Ned Deily, n...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
18-08-2009 o 22:32:55 Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. In Python 1.x/2.x 'print' is a keyword-based statement, not a function -- then you cannot use it in lambda (which in Python is limited to single expressions, and statements are not allowed in it). You can try using sys.stdout.write() instead. I want to avoid using a def if possible. But what for? Usualy def is more readable than lambda and it's not worth to lose readibility just to save a few keystrokes. Cheers, *j -- Jan Kaliszewski (zuo) z...@chopin.edu.pl -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Aug 18, 3:45 pm, Rami Chowdhury rami.chowdh...@gmail.com wrote: Lambda expressions are, I believe, syntactically limited to a single expression -- no statements, like 'print' is in Python 2.x. If you are strongly against just defining a function, you might have to use a trick to get around it -- this page (http://p-nand-q.com/python/stupid_lambda_tricks.html) has some suggestions. On Tue, 18 Aug 2009 13:32:55 -0700, Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. -- Rami Chowdhury Never attribute to malice that which can be attributed to stupidity -- Hanlon's Razor 408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD) The example I gave earlier is a bit contrived, the real example fundamentally requires a lambda since I am actually passing in local variables into the functions the lambda is wrapping. Example: def MyFunction(): localVariable = 20 CreateTask( lambda: SomeOtherFunction( localVariable ) ) # CreateTask () executes the functor internally This is more or less like the real scenario I'm working with. There are other (more personal) reasons why I prefer to avoid 'def' in this case. I want to keep the functor as central to the code that needs it as possible to improve code readability. Thanks for the help everyone. I guess in Python 3.0 the print() function will not require the import from __future__ to work in this particular case? -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Aug 18, 3:51 pm, Jan Kaliszewski z...@chopin.edu.pl wrote: 18-08-2009 o 22:32:55 Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. In Python 1.x/2.x 'print' is a keyword-based statement, not a function -- then you cannot use it in lambda (which in Python is limited to single expressions, and statements are not allowed in it). You can try using sys.stdout.write() instead. I want to avoid using a def if possible. But what for? Usualy def is more readable than lambda and it's not worth to lose readibility just to save a few keystrokes. Cheers, *j -- Jan Kaliszewski (zuo) z...@chopin.edu.pl I posted a bit earlier than you did. See my previous post. Thanks for the help. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On Tue, Aug 18, 2009 at 4:27 PM, Robert Daileyrcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? You can't, at least not with that example. Lambdas are restricted to a single expression[1]. Print is not an expression, it's a statement[2]. I'm guessing that your use case is not really in wrapping a print statement in an anonymous function. Given the first part of your message, you might find something of use in the functools module, particularly functools.partial [3]. 1: http://docs.python.org/tutorial/controlflow.html#lambda-forms 2: http://docs.python.org/reference/simple_stmts.html 3: http://docs.python.org/library/functools.html#functools.partial -- Jerry -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
why am I able to use print as a function in general-purpose code in my Python 2.6 script I believe it's because that is parsed as the print statement followed by a parenthesized expression. On Tue, 18 Aug 2009 13:42:59 -0700, Robert Dailey rcdai...@gmail.com wrote: On Aug 18, 3:40 pm, Chris Rebert c...@rebertia.com wrote: On Tue, Aug 18, 2009 at 1:32 PM, Robert Daileyrcdai...@gmail.com wrote: On Aug 18, 3:31 pm, Duncan Booth duncan.bo...@invalid.invalid wrote: Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? def print1(): print Foobar It looks like in your version of Python print isn't a function. It always helps if you say the exact version you are using in your question as the exact answer you need may vary. I'm using Python 2.6. And using the legacy syntax in the lambda does not work either. I want to avoid using a def if possible. Thanks. ch...@morpheus ~ $ python Python 2.6.2 (r262:71600, May 14 2009, 16:34:51) [GCC 4.0.1 (Apple Inc. build 5484)] on darwin Type help, copyright, credits or license for more information. print1 = lambda: print( Foobar ) File stdin, line 1 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax from __future__ import print_function print1 = lambda: print( Foobar ) print1() Foobar Cheers, Chris --http://blog.rebertia.com I see what you're saying now. However, why am I able to use print as a function in general-purpose code in my Python 2.6 script, like so: def SomeFunction(): print( Hello World ) But, I am not able to do this: SomeFunction = lambda: print( Hello World ) ?? Doesn't make sense. -- Rami Chowdhury Never attribute to malice that which can be attributed to stupidity -- Hanlon's Razor 408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD) -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
On 2009-08-18, Robert Dailey rcdai...@gmail.com wrote: Hello, I want to simply wrap a function up into an object so it can be called with no parameters. The parameters that it would otherwise have taken are already filled in. Like so: print1 = lambda: print( Foobar ) print1() However, the above code fails with: File C:\IT\work\distro_test\distribute_radix.py, line 286 print1 = lambda: print( Foobar ) ^ SyntaxError: invalid syntax How can I get this working? Doesn't this do what you want? def print1(): return print( Foobar ) -- Grant Edwards grante Yow! I want my nose in at lights! visi.com -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
The example I gave earlier is a bit contrived, the real example fundamentally requires a lambda since I am actually passing in local variables into the functions the lambda is wrapping. Example: def MyFunction(): localVariable = 20 CreateTask( lambda: SomeOtherFunction( localVariable ) ) # CreateTask () executes the functor internally This is more or less like the real scenario I'm working with. There are other (more personal) reasons why I prefer to avoid 'def' in this case. I want to keep the functor as central to the code that needs it as possible to improve code readability. what about def MyFunction(): localVariable = 20 def wrapper(): return SomeOtherFunction( localVariable ) CreateTask( wrapper ) the wrapper is only visible inside MyFunction. Leonhard -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
The example I gave earlier is a bit contrived, the real example fundamentally requires a lambda since I am actually passing in local variables into the functions the lambda is wrapping. Example: funcs = [] for i in xrange(10): def f(i=i): print i funcs.append(f) for f in funcs: f() This is more or less like the real scenario I'm working with. There are other (more personal) reasons why I prefer to avoid 'def' in this case. I want to keep the functor as central to the code that needs it as possible to improve code readability. Didn't you say the other day you came from C++? Given the galactic amount of hoops to jump through that language has to offer, typing def f instead of the even longe lambda strikes me as rather peculiar. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: How to create functors?
Dnia 18-08-2009 o 22:51:19 Robert Dailey rcdai...@gmail.com napisał(a): The example I gave earlier is a bit contrived, the real example fundamentally requires a lambda since I am actually passing in local variables into the functions the lambda is wrapping. Example: def MyFunction(): localVariable = 20 CreateTask( lambda: SomeOtherFunction( localVariable ) ) # CreateTask () executes the functor internally Lambda in Python is a sintactic sugar for some simple situations. But you *always* can replace it with def, e.g.: def MyFunction(): localVariable = 20 def TaskFunction(): SomeOtherFunction(localVariable) CreateTask(TaskFunction) If we say about You can also use functools.partial: import functools def MyFunction(): localVariable = 20 CreateTask(functools.partial(SomeOtherFunction, localVariable) ...which (especially) makes sense if passed function is supposed to be callend many times. This is more or less like the real scenario I'm working with. There are other (more personal) reasons why I prefer to avoid 'def' in this case. I want to keep the functor as central to the code that needs it as possible to improve code readability. IMHO def is mostly more readable (see my previous mail...). Thanks for the help everyone. I guess in Python 3.0 the print() function will not require the import from __future__ to work in this particular case? Print as a function is a standard feature of Py 3.x so it doesn't require it (see: http://docs.python.org/3.1/whatsnew/3.0.html ). Regards, *j -- Jan Kaliszewski (zuo) z...@chopin.edu.pl -- http://mail.python.org/mailman/listinfo/python-list
