Name bindings for inner functions.

[trevor_morgan]

The following code:

def functions():
l=list()
for i in range(5):
def inner():
return i
l.append(inner)
return l
print [f() for f in functions()]
returns [4,4,4,4,4], rather than the hoped for [0,1,2,3,4]. I presume
this is something to do with the variable i getting re-bound every time
we go through the loop, or something, but I'm not sure how to fix this.
I've found the ability in python to create functions on the fly and
store and index them for later use incredibly powerful, so it'd be nice
to get past this little roadblock.

[Andrea Griffini]

tr***********@yahoo.com wrote:

The following code:

def functions():
l=list()
for i in range(5):
def inner():
return i
l.append(inner)
return l
print [f() for f in functions()]
returns [4,4,4,4,4], rather than the hoped for [0,1,2,3,4]. I presume
this is something to do with the variable i getting re-bound every time
we go through the loop, or something, but I'm not sure how to fix this.

The problem is that "i" inside the function is indeed
the same variable for all the functions (the one you're
using for looping).

If you want a different variable for each function
you can use the somewhat ugly but idiomatic

def functions():
l=list()
for i in range(5):
def inner(i=i):
return i
l.append(inner)
return l

this way every function will have its own "i" variable,
that is initialized with the value of the loop variable
when executing the "def" statement.

Andrea

[trevor_morgan]

Thanks, that's exactly what I needed.
Andrea Griffini wrote:

tr***********@yahoo.com wrote:

The following code:

def functions():
l=list()
for i in range(5):
def inner():
return i
l.append(inner)
return l
print [f() for f in functions()]
returns [4,4,4,4,4], rather than the hoped for [0,1,2,3,4]. I presume
this is something to do with the variable i getting re-bound every time
we go through the loop, or something, but I'm not sure how to fix this.

The problem is that "i" inside the function is indeed
the same variable for all the functions (the one you're
using for looping).

If you want a different variable for each function
you can use the somewhat ugly but idiomatic

def functions():
l=list()
for i in range(5):
def inner(i=i):
return i
l.append(inner)
return l

this way every function will have its own "i" variable,
that is initialized with the value of the loop variable
when executing the "def" statement.

Andrea

[Fredrik Lundh]

tr***********@yahoo.com wrote:

The following code:

def functions():
l=list()
for i in range(5):
def inner():
return i
l.append(inner)
return l
print [f() for f in functions()]
returns [4,4,4,4,4], rather than the hoped for [0,1,2,3,4]. I presume
this is something to do with the variable i getting re-bound every time
we go through the loop

free variables bind to *names*, not objects. all your functions will
refer to the name "i" in "function"'s scope, which is bound to a 4 when
the loop has finished.

you can use the default argument mechanism to explicitly bind to an
object instead of a name:

def functions():
l=list()
for i in range(5):
def inner(i=i):
return i
l.append(inner)
return l

</F>

[James Stroud]

Andrea Griffini wrote:

tr***********@yahoo.com wrote:

>The following code:

def functions():
l=list()
for i in range(5):
def inner():
return i
l.append(inner)
return l
print [f() for f in functions()]
returns [4,4,4,4,4], rather than the hoped for [0,1,2,3,4]. I presume
this is something to do with the variable i getting re-bound every time
we go through the loop, or something, but I'm not sure how to fix this.

The problem is that "i" inside the function is indeed
the same variable for all the functions (the one you're
using for looping).

If you want a different variable for each function
you can use the somewhat ugly but idiomatic

def functions():
l=list()
for i in range(5):
def inner(i=i):
return i
l.append(inner)
return l

this way every function will have its own "i" variable,
that is initialized with the value of the loop variable
when executing the "def" statement.

Andrea

Yet another way to skin the same cat, maybe even less ugly, depending on taste.

def make_inner(i):
def inner():
return i
return inner

def functions():
return [make_inner(i) for i in range(5)]

print [f() for f in functions()]

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/