Assign function outputs to variables in list using for-loop

Question

I thought my question should've been pretty common but I couldn't find the answer online. I have a function that takes in 2 arguments and has 2 outputs. I want to run a list of these 2 arguments through the function and assign the output to the element in the list. Let me explain, the 2 arguments are in these 2 lists:

Lengths = [self.xS, self.yS, self.zS, self.Len1, self.Len2, self.Thick]
LengthsUnit = [self.xSUnit, self.ySUnit, self.zSUnit, self.Len1Unit, self.Len2Unit, self.ThickUnit]

The for loop I have constructed to run these arguments through the function looks like this:

for length, unit in zip(Lengths, LengthsUnit):
    length, unit = ConvertToSIVarUnit(self, length, unit, 'meter', 'Length')

After completion, I want the things in the list (take self.xS for example) to be modified. So the output of the first print statement should be different from the last:

Lengths = [self.xS, self.yS, self.zS, self.Len1, self.Len2, self.Thick]
LengthsUnit = [self.xSUnit, self.ySUnit, self.zSUnit, self.Len1Unit, self.Len2Unit, self.ThickUnit]

print(self.Xs)

for length, unit in zip(Lengths, LengthsUnit):
    length, unit = ConvertToSIVarUnit(self, length, unit, 'meter', 'Length')

print(self.xS)

But the output of the 2 print functions are the same so the for-loop-function combination might not be working.

Barmar · Accepted Answer · 2020-06-22 16:13:03Z

2

You need to assign to the list elements, not the variables that hold the list values. Use enumerate() to get the indexes.

for i, (length, unit) in enumerate(zip(Lengths, LengthsUnit)):
    Lengths[i], LengthsUnit[i] = ConvertToSIVarUnit(self, length, unit, 'meter', 'Length')

Things would probably be easier if you didn't keep the lengths and units in different lists, though. Use a list of dictionaries or tuples to keep the values and units together.

It's not possible for changing the list to affect the original object attributes. You could use a list of attribute names rather than values, then use getattr() and setattr.

Lengths = ['xS', 'yS', 'zS', 'Len1', 'Len2', 'Thick']
LengthsUnit = ['xSUnit', 'ySUnit', 'zSUnit', 'Len1Unit', 'Len2Unit', 'ThickUnit']
for length, unit in zip(Lengths, LengthsUnit):
    new_length, new_unit = ConvertToSIVarUnit(self, getattr(self, length), getattr(self, unit), 'meter', 'Length')
    setattr(self, length, new_length)
    setattr(self, unit, new_unit)

print(self.xS)

edited Jun 22, 2020 at 16:13

answered Jun 22, 2020 at 15:58

Barmar

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4 Comments

Austin Benny Over a year ago

Thanks for answering. I still want to reference say for example self.xS instead of Lengths[0] every time I want the use self.xS. Is there a way to do this inside the loop or should I look for try something else?

Barmar Over a year ago

Once you put a value inside a list, it's no longer related to the original location that it came from.

Barmar Over a year ago

Python doesn't have anything like C pointers that can refer to attributes of other objects.

Barmar Over a year ago

I've added code that puts the attribute names in the lists to allow indirecting.

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