[New-bugs-announce] [issue8283] series of lamdas in loop sets the paramater on all calls to that of the last call

alan hoover report at bugs.python.org
Thu Apr 1 20:26:02 CEST 2010


New submission from alan hoover <alan.hoover at gmail.com>:

Background:
building a screen using Tkinter based on information from a database to let user take action on the database rows.  I don't know how many rows there will be, so I'm storing the widgets in arrays for each column.

Code:
for row in getDBrows():
    self.buttons.append(Tkinter.Button(self,text='UnLoad it', command=lambda :self.unload(row[0])))

Problem:
When executing the above code, all the buttons have the key to the database table that belongs to the last row.  I found a work around -- by moving the call to create the button (containing the lambda) to a separate function and call that function to create the button instead of creating the button directly, the buttons then make their callback with a correct database key.

Workaround:
for row in getDBrows():
    self.buttons.append(self.buildbutton(row[0]))
.
.
def buildbutton(self,key):
    return Tkinter.Button(self,text='UnLoad it', command=lambda: self.unload(key))

When using the workaround code instead of the original code, the button
for each row has the appropriate key to the database.

Speculation:
It acts like the lambda definitions don't get solidified until the containing block exits; at that time, the lambda definition(s) get locked in with the current value of the variable getting passed into the lambda as a parameter.  By moving the lambda call to a different block (separate function), the lambda gets "locked" when that block (function) exits instead of when the containing block (loop) exits.

----------
components: IDLE, Tkinter, Windows
messages: 102120
nosy: alan.hoover
severity: normal
status: open
title: series of lamdas in loop sets the paramater on all calls to that of the last call
type: behavior
versions: Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue8283>
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