How do I append a lambda to a list in python?

Question

I am trying to make a program that creates a list of lambda functions of the format y=mx+b, where 'm' and 'b' are predetermined values

My overall goal is to implement a function that

• Takes a picture
• Finds the lines on it
• Extends them across the whole picture in a solid colour

Basically, something like a Hough transforms if you know what that is.

Once I have the lines for a specific image, I can create a lambda function to represent the slope of the line and where it begins. I'm having an issue not being able to append a lambda function to the list.

I have tried this :

if __name__ == "__main__":
  nums = []
  for i in range(10):
    j = lambda x: x + i
    nums.append(j)
  for i in nums:
    print(i(1))

Here is the error I'm getting :

Traceback (most recent call last):
  File "C:/Users/me/.PyCharmCE2018.3/config/scratches/scratch_3.py", line 7, in <module>
    print(i(1))
  File "C:/Users/me/.PyCharmCE2018.3/config/scratches/scratch_3.py", line 4, in <lambda>
    j = (lambda x: x + i)
TypeError: unsupported operand type(s) for +: 'int' and 'function'

Answer 1

The problem is that the lambdas you create are referring to the current value of i in the active stack frame. When you later reuse i for the second for loop, it is bound to the lambdas in your list. When invoked as i(1), the lambdas are trying to evaluate 1 + i where i is the lambda, so of course you get an error.

Probably what you want is to freeze the value of i at the point at which the lambda is created. You can do this by replacing:

j = lambda x: x + i

with:

j = (lambda y: lambda x: x + y)(i)

This effectively captures the current value of i by binding it to a lambda variable, then immediately applying that lambda, after which the binding remains fixed.

Answer 2

This will give you a clue:

>>> i=1
>>> a=lambda x:x+i
>>> a(5)
6
>>> i=2
>>> a(5)
7

lambda uses i in the outer scope. In the OP case, all the functions are the same. Using i in the final loop makes i a function, not an integer. Change it to something else, and you'll find the functions are all the same, using the last value of i:

nums = []
for i in range(10):
    j = lambda x: x + i
    nums.append(j)
for f in nums:
    print(f(1))

10
10
10
10
10
10
10
10
10
10

The fix is, make i a parameter to the function, to capture the value as a local variable:

nums = []
for i in range(10):
    j = lambda x,i=i: x + i
    nums.append(j)
for f in nums:
    print(f(1))

1
2
3
4
5
6
7
8
9
10

Answer 3

Your value of i has changed and it's not what you think.

First you create lambda:

j = lambda x: x + i

in hope, that i will remain as CURRENT value (so 0, 1, 2 and so on).

Then you execute it:

print(i(1))

Do you see, how you named your second iterator variable i? Change it to j and your example will work. Why? Because python resolves value of i in your lambda, when you execute it, not when you define it. So when you execute your lambda (i(1)) it will go to your lambda body and try x + i. Then it will lookup i, which now contains your lambda (not the INTEGER!). Hence your problem.

You need to do double function to make it work properly. Try this:

if __name__ == "__main__":
  nums = []
  for i in range(10):
    def generate_lambda(i):
      return lambda x: x + i
    j = generate_lambda(i)
    nums.append(j)
  for i in nums:
    print(i(1))

Why does this work? When you call generate_lambda, there will be i variable with your INTEGER value. It will shadow variable i used later on to iterate over lambdas. And since you never modify i variable inside generate_lambda function, it will stay like this forever.

Answer 4

I think you need to learn something more about lambda functions... Actually, it's syntax is like : [lambda arguments: expression] So, the issue is you have two variables in the expression, so you need to pass two arguments. I don't really get what you want to achieve by this function, but I guess you need to have two arguments for m and b.

In your code, you need to initialize x and pass it as an argument to lambda.

            nums = []
            x=0
            for i in range(10):
                j = lambda x,i : x + i
                nums.append(j)
            for i in nums:
                print(i(1,1))

Answer 5

You can use operator.add and functools.partial and do not lambda at all:

import operator
import functools


if __name__ == "__main__":
    nums = []

    for i in range(10):
        nums.append(functools.partial(operator.add, i))
    for i in nums:
        print(i(1))