What's going on here? I'm trying to create a list of functions:
def f(a,b):
return a*b
funcs = []
for i in range(0,10):
funcs.append(lambda x:f(i,x))
This isn't doing what I expect. I would expect the list to act like this:
funcs[3](3) = 9
funcs[0](5) = 0
But all the functions in the list seem to be identical, and be setting the fixed value to be 9:
funcs[3](3) = 27
funcs[3](1) = 9
funcs[2](6) = 54
Any ideas?
Python call function inside for loop A Python for loop is used to loop through an iterable object (like a list, tuple, set, etc.) and perform the same action for each entry. We can call a function from inside of for loop.
Accepted Answer "function" as a keyword is only used for defining functions, and cannot be used inside a loop (or inside an "if" or "switch" or other control statement.) The only kinds of functions that can be defined within loops are anonymous functions.
Answer. Yes, you can use a function call in the while expression. If calling only a function in the expression, it should return True or False . If the function is part of a more complex expression, then the end result of the expression should evaluate to True or False .
Python for loop components In general, any object that supports Python's iterator protocol can be used in a for loop. A variable that holds each element from the container/sequence/generator.
lambdas in python are closures.... the arguments you give it aren't going to be evaluated until the lambda is evaluated. At that time, i=9 regardless, because your iteration is finished.
The behavior you're looking for can be achieved with functools.partial
import functools
def f(a,b):
return a*b
funcs = []
for i in range(0,10):
funcs.append(functools.partial(f,i))
Yep, the usual "scoping problem" (actually a binding-later-than-you want problem, but it's often called by that name). You've already gotten the two best (because simplest) answers -- the "fake default" i=i
solution, and functools.partial
, so I'm only giving the third one of the classic three, the "factory lambda":
for i in range(0,10):
funcs.append((lambda i: lambda x: f(i, x))(i))
Personally I'd go with i=i
if there's no risk of the functions in funcs
being accidentally called with 2 parameters instead of just 1, but the factory function approach is worth considering when you need something a little bit richer than just pre-binding one arg.
There's only one i
which is bound to each lambda, contrary to what you think. This is a common mistake.
One way to get what you want is:
for i in range(0,10):
funcs.append(lambda x, i=i: f(i, x))
Now you're creating a default parameter i
in each lambda closure and binding to it the current value of the looping variable i
.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With