Python change string into variable name

Question

So I have a list of strings MyList=['ID_0','ID_1','ID_2',.....] from which I used an exec function to create a empty list with elements as names i.e. each I have empty lists ID_0=[] and so on. I now want to create a for loop where I take each list by name and append elements to it like

for i in range(len(MyList)):
     *magic*
     ID_0.append(i)

where with each loop it uses to next list i.e. ID_1,ID_2.... and so on

I wanted to take each string in MyList and change it into a variable name. Like take 'ID_0' and change it into say ID_0 (which is a empty list as defined earlier) and then make a list BigList=[ID_0,ID_1,ID_2,.....] so that I can just call each list from BgList in my for loop but I dont know how

You want this: docs.python.org/3/c-api/reflection.html. Reflection is problematic to describe unless you already know what it's called, :) — nicomp
– nicomp, Commented Jul 30, 2021 at 13:04
Well, just about everything so far screams “this is a terrible approach”, but as a nudge, why can’t you keep using exec? — Rich L
– Rich L, Commented Jul 30, 2021 at 13:04
It looks like you rather need a dictionary with ID_0, ID_1 as keys and set their values to said list. — Tranbi
– Tranbi, Commented Jul 30, 2021 at 13:05

hugovdberg · Accepted Answer · 2021-07-30 13:19:49Z

5

A safer way instead of using exec might be to construct a dictionary, so you can safely refer to each list by its ID:

MyList=['ID_0','ID_1','ID_2']

lists = {}
for i, list_id in enumerate(MyList):
    lists[list_id] = [i]

Note that using enumerate is more pythonic than range(len(MyList)). It returns tuples of (index, item).

Alternatively, you could also use defaultdict, which constructs a new item in the dictionary the first time it is referenced:

from collections import defaultdict

lists = defaultdict(list)
for i, list_id in enumerate(MyList):
    lists[list_id].append(i)

answered Jul 30, 2021 at 13:19

hugovdberg

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