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Why doesn't dictionary value change accordingly in the original dict when it's modified?

March 30, 2022

import pandas as pd

df1 = pd.DataFrame({
  "a": [420, 380, 390],
  "b": [50, 40, 45]
})

df2 = pd.DataFrame({
  "c": [420, 380, 390],
  "d": [50, 40, 45]
})

dfs = {'df1': df1, 'df2':df2}

for name, df in dfs.items():
    df = {}
    pass

print('dfs: ', dfs)

When I modify each df in the for loop, why isn’t this change updated to the dfs? I suppose the print() should print an empty dict, but it didn’t change at all. I have do an explicit reassignment to update dfs:

for name, df in dfs.items():
   df = {}
   dfs[name] = df

>Solution :

df is a local variable defined each time for loop runs. So, it would behave like a new variable, and assigning a new value would not change the source (dfs). What you need is exactly as your second cell, but to have a better understanding, I have edited it a little bit:

import pandas as pd

df1 = pd.DataFrame({
  "a": [420, 380, 390],
  "b": [50, 40, 45]
})

df2 = pd.DataFrame({
  "c": [420, 380, 390],
  "d": [50, 40, 45]
})

dfs = {'df1': df1, 'df2':df2}

print('dfs before: ', dfs)
for name, df in dfs.items():
    dfs[name] = {}


print('dfs after: ', dfs)

Output

dfs before:  {'df1':      a   b
0  420  50
1  380  40
2  390  45, 'df2':      c   d
0  420  50
1  380  40
2  390  45}
dfs after:  {'df1': {}, 'df2': {}}