473,837 Members | 1,622 Online
Bytes | Software Development & Data Engineering Community
+ Post

Home Posts Topics Members FAQ

Problem understanding how closures work

....at least, I think that I'm having a problem understanding the way
closures work.

I'm trying to define a function for an object which will take certain
objects from the parent scope at the time that function is defined.
For some reason, if I do this function definition in a loop, the
locals given by that function (is this a closure?) are changed each
iteration of the loop, whereas if the function definition is isn't
looped over, I get the behavior I desire. Can anyone provide any
insight for me?

thanks,
-tom!

First, the output:

Test 1 doesn't work the way I would expect:
Test 4 says, "Test 0"
Test 4 says, "Test 1"
Test 4 says, "Test 2"
Test 4 says, "Test 3"
Test 4 says, "Test 4"

....but test 2 does?
Test 0 says, "Test 0"
Test 1 says, "Test 1"
Test 2 says, "Test 2"
Test 3 says, "Test 3"
Test 4 says, "Test 4"

Next, the program:

class Test:
def __init__(self, name):
self.name = name

def DoCall(self):
self.ExternalCa ll(self.name)

# The first test.
def CreateTests1(co unt):
tests = []
for i in xrange(count):
name = 'Test %d' % i
t = Test(name)
tests.append(t)

def ExCall(text):
print '%s says, "%s"' % (name, text)

t.ExternalCall = ExCall

return tests

# The second test.
def CreateTests2(co unt):
tests = []
for i in xrange(count):
t = CreateTest(i)
tests.append(t)
return tests

def CreateTest(inde x):
name = 'Test %d' % index
t = Test(name)

def ExCall(text):
print '%s says, "%s"' % (name, text)

t.ExternalCall = ExCall
return t

print 'Test 1 doesn\'t work the way I would expect:'
for t in CreateTests1(5) :
t.DoCall()

print '\n...but test 2 does?'
for t in CreateTests2(5) :
t.DoCall()
Dec 12 '06 #1
3 1363
Tom Plunket wrote in news:cm******** *************** *********@4ax.c om in
comp.lang.pytho n:
...at least, I think that I'm having a problem understanding the way
closures work.

I'm trying to define a function for an object which will take certain
objects from the parent scope at the time that function is defined.
For some reason, if I do this function definition in a loop, the
locals given by that function (is this a closure?) are changed each
iteration of the loop, whereas if the function definition is isn't
looped over, I get the behavior I desire. Can anyone provide any
insight for me?
Test 1 doesn't work the way I would expect:
Test 4 says, "Test 0"
Test 4 says, "Test 4"
def CreateTests1(co unt):
tests = []
for i in xrange(count):
name = 'Test %d' % i
t = Test(name)
tests.append(t)

def ExCall(text):
print '%s says, "%s"' % (name, text)

t.ExternalCall = ExCall

return tests
"name" in the above code is bound to a an entry in "CreateTests1"' s
locals, and ExCall has a (hidden) reference to that locals, so
by the time ExCall is finally called the value associated
with "name" has been replaced by (count - 1).

The solution (as always) is to add another level of indirection:

def create_tests( count ):
def make( arg ):
def ExCall( text ):
print arg, text
return ExCall

tests = []

for i in range( count ):
name = i
t = Test( name )
t.ExternalCall = make( name )

In the above, every call to make() creates a new frame (a new set
of locals) and binds the value of the passed in "name" to the
name "arg" in this new frame, it will be this value that is
eventually printed.

There is a trick with default arguments that lets you do
what you want with a bit less faffing about:
>>r = []
for i in range(10):
def f( i = i ):
print i
r.append( f )
>>for i in r:
i()

In this example the value of "i" is bound to the default argument
for the function "f" every time the def f() statments are executed.

Rob.
--
http://www.victim-prime.dsl.pipex.com/
Dec 12 '06 #2
On 12 dic, 17:23, Tom Plunket <t...@fancy.org wrote:
...at least, I think that I'm having a problem understanding the way
closures work.

I'm trying to define a function for an object which will take certain
objects from the parent scope at the time that function is defined.
def CreateTests1(co unt):
tests = []
for i in xrange(count):
name = 'Test %d' % i
t = Test(name)
tests.append(t)

def ExCall(text):
print '%s says, "%s"' % (name, text)

t.ExternalCall = ExCall

return tests
name, inside ExCall, is a free variable. Python builds a closure
including the string whose name is "name" in the enclosing scope. Not
the *value* which happens to have at this momment. When you execute
ExCall, the reference to name yields its last, current, value.
If you want "the value at the moment the function is created" you can
use a default argument:

def ExCall(text, name=name): ...

Your second test works because you don't modify "name" between the
original definition and its execution.

--
Gabriel Genellina

Dec 12 '06 #3
Rob Williscroft wrote:
"name" in the above code is bound to a an entry in "CreateTests1"' s
locals, and ExCall has a (hidden) reference to that locals, so
by the time ExCall is finally called the value associated
with "name" has been replaced by (count - 1).
Ah, I got it. Thanks. Thanks too to Gabriel.
-tom!
Dec 12 '06 #4

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

Similar topics

4
4817
by: Derek | last post by:
Hi, I've built a rather large CGI that dumps a lot of data and a fairly complex javascript app out to the client's browser. Granted this may be poor style according to someone web design philosophy but that is the way things need to work for now here. The problem I'm having is that it appears that the browsers (IE, mozilla and netscape) are sometimes getting confused about wether the javascript code is running. By this I mean when I...
8
2573
by: rdlebreton | last post by:
Hi, Folks! I've been trying to develop my own version of these draggable layers and I have been limiting myself to IE6...for now. I have looked at some other examples to get ideas of creating an alternative to pop-up windows in a web page. The code I have works (sort of). The problem is that I can move these layers around when I move the mouse slowly, however, if I move it fast the web browser looses track of the motion and leaves the...
3
1279
by: Fabio Cavassini | last post by:
I'm new to JavaScript and this is annoying me. I have defined a "class" (JavaScript OO seems really strange to me...) in the following way: function StateSuggestions(pSource) { this.source = pSource; this.req=new ActiveXObject("Msxml2.XMLHTTP"); //this.req != null this.Process = MyProcess; this.requestSuggestions = MyRequestSuggestions; ....
6
2266
by: Dasn | last post by:
Hi, there. 'lines' is a large list of strings each of which is seperated by '\t' I wanna split each string into a list. For speed, using map() instead of 'for' loop. 'map(str.split, lines)' works fine , but... when I was trying: I got "TypeError: 'list' object is not callable".
5
1730
by: Leo Meyer | last post by:
Hello, somewhere I have read that JavaScript supports closures. Does someone know how to make them work? What I want to do is this: function f1(x, obj) { var eventhandler = function(event) {
4
8088
by: Dan Michael Heggå | last post by:
Hi, I've working on inplace-editing. My problem is the following: This works in all browsers: myDiv.innerHTML = "<p>...</p>"; but this doesn't work in IE: myDiv.innerHTML = "<form>...</form>";
1
1830
by: alain | last post by:
Hi, I have a problem with closures. I am trying to implement yet another design by contract decorator which would look like the following: <pre> def contract(f): def newf(*args, **kw): import new precondition = new.function(f.func_code.co_consts,
4
1458
by: king kikapu | last post by:
Hi, i am trying, to no avail yet, to take a C#'s overloaded functions skeleton and rewrite it in Python by using closures. I read somewhere on the net (http://dirtsimple.org/2004/12/python-is- not-java.html) that in Python we can reduce code duplication for overloaded functions by using closures. I do not quite understand this. Let's say we have the following simple C# code:
26
2822
by: Aaron \Castironpi\ Brady | last post by:
Hello all, To me, this is a somewhat unintuitive behavior. I want to discuss the parts of it I don't understand. .... f= lambda: n .... 9 9
0
10562
jinu1996
by: jinu1996 | last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth. The Art of Business Website Design Your website is...
1
10617
by: Hystou | last post by:
Overview: Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
0
10263
tracyyun
by: tracyyun | last post by:
Dear forum friends, With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
1
7803
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules. He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms. Adolph will...
0
6989
by: conductexam | last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one. At the time of converting from word file to html my equations which are in the word document file was convert into image. Globals.ThisAddIn.Application.ActiveDocument.Select();...
0
5666
by: TSSRALBI | last post by:
Hello I'm a network technician in training and I need your help. I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs. The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols. I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
1
4469
by: 6302768590 | last post by:
Hai team i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
2
4036
muto222
by: muto222 | last post by:
How can i add a mobile payment intergratation into php mysql website.
3
3122
bsmnconsultancy
by: bsmnconsultancy | last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.