473,574 Members | 3,066 Online

# questions about functions inside a function

I want to create a list of function.

Here is my code:
In [9]: a = []

In [10]: for i in range(4):
....: b = lambda : i**2
....: a.append(b)
....:
....:

In [11]: for f in a:
....: f()
....:
....:
9
9
9
9

What I want is, the value of i should be bounded to the anonymous function.
And the output should like this:

for f in a:
print f()
0
1
4
9

How to achieve this?

Thanks a lot!
Jul 16 '07 #1
5 1218
"fd********@gma il.com" <fd********@gma il.comwrote:
What I want is, the value of i should be bounded to the anonymous
function. And the output should like this:

for f in a:
print f()
0
1
4
9

How to achieve this?
Use default arguments when defining your function: default arguments are
evaluated when the function is defined, bound arguments get the current
value of the variable at the point when it is referenced during the call.

Also, since you give your function a name anyway ('b') you might as well
use a named function as being clearer than using lambda:
>>a = []
for i in range(4):
def b(i=i):
return i**2
a.append(b)

>>for f in a:
f()
0
1
4
9
>>>
Jul 16 '07 #2
I want to create a list of function.

Here is my code:
In [9]: a = []

In [10]: for i in range(4):
....: b = lambda : i**2
....: a.append(b)
....:
....:

In [11]: for f in a:
....: f()
....:
....:
9
9
9
9

What I want is, the value of i should be bounded to the anonymous
function. And the output should like this:

for f in a:
print f()
0
1
4
9

How to achieve this?
>>a = [lambda i=i:i**2 for i in range(4)]
for f in a:
.... print f()
....
0
1
4
9
>>>
HTH
Jul 16 '07 #3
On Jul 16, 4:49 am, "fdu.xia...@gma il.com" <fdu.xia...@gma il.com>
wrote:

(snipped)
>
What I want is, the value of i should be bounded to the anonymous function.
And the output should like this:

for f in a:
print f()
0
1
4
9

How to achieve this?

Thanks a lot!

The functools module, in Python 2.5 allows:
>>import functools
square = lambda i: i ** 2
anon = [functools.parti al(square, i) for i in range(4)]
for f in anon:
.... print f()
....

--
Hope this helps,
Steven

Jul 16 '07 #4
fd********@gmai l.com wrote:
What I want is, the value of i should be bounded to the anonymous
function. And the output should like this:
....
How to achieve this?
clearer) way to do such things is something like

class MyFunc(object):
"""A function object."""
def __init__(self, val):
self.val = val

def __call__(self):
"""Return value squared"""
return self.val**2

a = []
for i in range(4):
a.append(MyFunc (i))

for f in a:
f()
Jeremy

--
Jeremy Sanders
http://www.jeremysanders.net/
Jul 16 '07 #5
Jeremy Sanders a écrit :
fd********@gmai l.com wrote:

>>What I want is, the value of i should be bounded to the anonymous
function. And the output should like this:

...
>>How to achieve this?

clearer) way to do such things is something like

class MyFunc(object):
"""A function object."""
def __init__(self, val):
self.val = val

def __call__(self):
"""Return value squared"""
return self.val**2

a = []
for i in range(4):
a.append(MyFunc (i))

for f in a:
f()
I wouldn't say it's "clearer" - at least in this concrete use case. And
it takes more than just defining the __call__ method to have a fully
function-like object (hint: try using a MyFunc instance as a method).
Jul 16 '07 #6

This thread has been closed and replies have been disabled. Please start a new discussion.

### Similar topics

 0 2335 by: Anthony Baxter | last post by: To go along with the 2.4a3 release, here's an updated version of the decorator PEP. It describes the state of decorators as they are in 2.4a3. PEP: 318 Title: Decorators for Functions and Methods Version: \$Revision: 1.34 \$ Last-Modified: \$Date: 2004/09/03 09:32:50 \$ Author: Kevin D. Smith, Jim Jewett, Skip Montanaro, Anthony Baxter 4 2131 by: Chuck Ritzke | last post by: I keep asking myself this question as I write class modules. What's the best/smartest/most efficient way to send a large object back and forth to a class module? For example, say I have a data access module that creates a large disconnected dataset from a database. I want to pass that dataset back to the calling program. And then perhaps I... 15 1994 by: Pelle Beckman | last post by: Hi all, I have a few newbie questions: In function declaration what does a 'const' mean inside the parameter list ? That it won't modify the value? void MemberFunction(const int x); 4 1824 by: Rhino | last post by: I've been playing with Java UDFs for the last couple of days and I've got some questions about scratchpads. I'm running DB2 LUW V8 (FP8) on WinXP. Somewhere in the manuals, I found some remarks that said I could either manage my scratchpad with the getScratchpad() and setScratchpad() methods *OR* set up my own class variables to keep the... 15 1777 by: copx | last post by: Q1: If an array is declared static in file A is it still valid to access it from file B? I mean if a function form file A which returns a pointer to a position inside of the array is called from file B can functions in file B read/write to the array using this pointer? I know that this works and doesn't produce a warning with every compiler... 9 1754 by: questions? | last post by: if I use malloc() in a function to allocate a space for my array of structures. I didn't free() them anywhere in the program. I found that I can still use that space after I come back to main(). Am I supposed to use that space or it is temporary and will be removed soon afterwards? Thanks 39 2488 by: Digital Puer | last post by: I'm not the world's greatest C++ programmer, so I had a hard time with these. Some help would be appreciated. 1. Comment on the declaration of function Bar() below: class Foo { static int Bar(int i) const; } (I said that since Bar is a static method, it can only access 6 2755 by: Venkatesh | last post by: Hello All, I have couple of doubts regarding the concept of on-demand javascript loading using javascript code. I could see on the net different techniques for achieving this - techniques like: 1. document.write("); 2. sc = document.createElement("