Python: append list in for-loop unexpected result

Question

I am trying to create a new variable from a list ('provider') that checks if some ids are present in another column in the data frame:

import pandas as pd

xx = {'provider_id': [1, 2, 30, 8, 8, 7, 9]}
xx = pd.DataFrame(data=xx)

ids = [8,9,30]
names = ["netflix", "prime","sky"]

for id_,name in zip(ids,names):
    provider = []
    if id_ in xx["provider_id"]:
       provider.append(name)
provider

excpected result:

['netflix', 'prime', 'sky']

actual result:

['sky']

So the for loop keeps overwriting the result of name inside the loop? This functionality seems weird to me and I honestly don't know how to prevent this other then to write three individual if statements.

quamrana · Accepted Answer · 2020-03-21 16:11:09Z

4

Your loop keeps initialising the list. Move the list outside the loop:

provider = []
for id_,name in zip(ids,names):
    if id_ in xx["provider_id"]:
        provider.append(name)
print(provider)

answered Mar 21, 2020 at 16:11

quamrana

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Comments

gold_cy · Accepted Answer · 2020-03-21 16:16:26Z

1

Scrap the loops altogether and use the built-in pandas methods. It will work much faster.

df = pd.DataFrame({'ids': [8,9,30], 'names': ["netflix", "prime","sky"]})

cond = df.ids.isin(xx.provider_id)

df.loc[cond, 'names'].tolist()

['netflix', 'prime', 'sky']

answered Mar 21, 2020 at 16:16

gold_cy

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Comments

yatu · Accepted Answer · 2020-03-21 16:22:18Z

1

One way to make this more efficient is using sets and isin to find the matching ids in the dataframe, and then a list comprehension with zip to keep the corresponding names.

The error as @quamrana points out is that you keep resetting the list inside the loop.

s = set(xx.loc[xx.isin(ids).values, 'provider_id'].values)
# {8, 9, 30}
[name for id_, name in zip(ids, names) if id_ in s]
# ['netflix', 'prime', 'sky']

edited Mar 21, 2020 at 16:22

answered Mar 21, 2020 at 16:14

yatu

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