Replacing one method with another within a loop using lambda in Python

Question

Here is an example of what I am trying to achieve. The class has a someMethod which has to be replaced with some other method (in this case returnStuff) when instance is created. I am using lambda to replace the method as the new one has an argument. Running the replaced method from a loop causes no problem, but running it after the loop turns all arguments to the same value. Replacing methods manualy works ok though. The code prints out everything needed to understand the problem.

def returnStuff(stuff):
    return stuff


class SomeClass(object):
    def __init__(self):
        pass

    def someMethod(self):
        pass


classInstances = []

#RUN METHODS WITHIN CREATION LOOP
print 'STARTING CREATION LOOP:\n'

for i in range(1, 6):
    classInstance = SomeClass()

    classInstances.append(classInstance)

    classInstance.someMethod = (lambda: returnStuff(i))

    print 'I was returned from the Creation Loop and I am fabulous: ', classInstance.someMethod(), '\n'


print '====================================================================\n'
print 'RUN METHODS AFTER THE LOOP:\n'

#CALLING METHODS LATER ON
for inst in classInstances:
    print 'I was returned after creation, and I am not who I want to be: ', inst.someMethod(), '\n'

print '====================================================================\n'
print 'TRYING MANUAL APPROACH (NO LOOP):\n'

classInstance1 = SomeClass()
classInstance1.someMethod = (lambda: returnStuff(1))
print 'I was returned from the manually replaced method. I\'m pretty fine: ', classInstance1.someMethod(), '\n'

classInstance2 = SomeClass()
classInstance2.someMethod = (lambda: returnStuff(2))
print 'I was returned from the manually replaced method. I\'m pretty fine: ', classInstance2.someMethod(), '\n'

classInstance3 = SomeClass()
classInstance3.someMethod = (lambda: returnStuff(3))
print 'I was returned from the manually replaced method. I\'m pretty fine: ', classInstance3.someMethod(), '\n'

classInstance4 = SomeClass()
classInstance4.someMethod = (lambda: returnStuff(4))
print 'I was returned from the manually replaced method. I\'m pretty fine: ', classInstance4.someMethod(), '\n'

classInstance5 = SomeClass()
classInstance5.someMethod = (lambda: returnStuff(5))
print 'I was returned from the manually replaced method. I\'m pretty fine: ', classInstance5.someMethod(), '\n'

there are better ways to get a closure than with lambda – Chad S. Oct 23 '15 at 20:32 — Chad S., Oct 23 '15 at 20:32

Chad S. · Accepted Answer · 2015-12-01T00:56:04.450

Rather than using lambda (which doesn't properly create a closure for your i variable). You should just use partial or some other function factory.

E.g.

from functools import partial
for i in range(1, 6):
    classInstance = SomeClass()
    classInstances.append(classInstance)
    classInstance.someMethod = partial(returnStuff, i)

or

def methodFactory(n):
  def returnStuff():
    return n

for i in range(1, 6):
  classInstance = SomeClass()
  classInstances.append(classInstance)
  classInstance.someMethod = methodFactory(i)

Replacing one method with another within a loop using lambda in Python

1 Answers1