Function modifies the original dictionary even after the copy

Question

I perform a copy inside a function and return a new dictionary but it seems the original dictionary is also modified. Does anyone know why? and how to fix this:

Note: The input is dictionary of dataframe. This mean that each value is dictionary.

Here is an example:

def Testing(data,col1,col2):
  data2 = data.copy()
  for ii in data:
     data2[ii].drop([col1,col2],axis=1, inplace=True)

  return data2

Calling the above function like this:

Data_new = Testing(Data, col1,col2)

It seems both Data and Data_new have similar columns and are identical. Can anyone tell me why and how to fix it?

You're not making a deep copy, you're making a shallow copy. — Barmar, Jun 08 '22 at 18:31
`drop()` isn't a dictionary method. This code looks like it operates on a pandas dataframe. — Barmar, Jun 08 '22 at 18:32
Is `Data` a dictionary, a dataframe, or a dictionary of dataframes? Please [edit] to include a sample of your input and your current and expected output to make a [mcve] s that we can better understand your question — G. Anderson, Jun 08 '22 at 18:36
@G.Anderson, what do you mean confusing it? it is a dictionary of dataframe. hence it is a dictionary. Updated and added the type of input. — user59419, Jun 08 '22 at 18:39
Does this answer your question? [Understanding dict.copy() - shallow or deep?](https://stackoverflow.com/questions/3975376/understanding-dict-copy-shallow-or-deep) — TheFungusAmongUs, Jun 08 '22 at 18:42

score 1 · Answer 1 · answered Jun 08 '22 at 19:13

As someone mentioned in the comments, you are having a shallow copy vs deep copy problem. The copy method makes a shallow copy, while the deepcopy method makes a deep copy and completely separates the new and old dictionaries.

from copy import deepcopy
def Testing(data,col1,col2):
  data2 = deepcopy(data)
  for ii in data:
     data2[ii].drop([col1,col2],axis=1, inplace=True)
  return data2


Data_new = Testing(Data, col1,col2)

Function modifies the original dictionary even after the copy

1 Answers1