Question

I have the following:

class Trade:
    def __init__(self):
        entry = [0, 1, 2, 3]
        exit = [1, 2, 3, 4]
        met = [0, 0, 0, 0]
        self.stats = zip(entry, exit, met)



t = Trade()

for entry, exit, met in t.stats:
        met = 1

for entry, exit, met in t.stats:
    print(entry, exit, met)

I am expecting all values of met to be 1. But that is not the case. What is the reason for this, and how can I make it so that the modifications to met are reflected in the instance of Trade class t. Does the for statement produce a copy of t.stats rather than iterating through its original copy?

Answer 1

When you loop over the values of a list, the values are assigned to local variables. Changing those local variables will not change the original values stored in the list.

Use a enumerate() call to add indexes, then use that index to change the original list:

for i, (entry, exit, met) in enumerate(t.stats):
    t.stats[i] = (entry, exit, 1)