[FIXED] How do I append a lambda to a list in python?

Issue

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'

Solution

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.

Answered By - Tom Karzes

Answer Checked By - Candace Johnson (PHPFixing Volunteer)