OrderedDict contain list of DataFrame - pandas

Question

I don't understand why method1 is fine but not the second ...
method 1
import pandas as pd
import collections
d = collections.OrderedDict([('key', []), ('key2', [])])
df = pd.DataFrame({'id': [1], 'test': ['ok']})
d['key'].append(df)
d
OrderedDict([('key', [ id test
0 1 ok]), ('key2', [])])
method 2
l = ['key', 'key2']
dl = collections.OrderedDict(zip(l, [[]]*len(l)))
dl
OrderedDict([('key', []), ('key2', [])])
dl['key'].append(df)
dl
OrderedDict([('key', [ id test
0 1 ok]), ('key2', [ id test
0 1 ok])])
dl == d True

Answer 1

The issue stems from creating empty lists like so: [[]] * len(l) what this is actually doing is copying the reference to the empty list multiple times. So what you end up with is a list of empty lists that all point to the same underlying object. When this happens, any change you make to the underlying list via inplace operations (such as append) will change the values inside of all references to that list.
The same type issue comes about when assigning variables to one another:
a = []
b = a
# `a` and `b` both point to the same underlying object.
b.append(1) # inplace operation changes underlying object
print(a, b)
[1], [1]
To circumvent your issue instead of using [[]] * len(l) you can use a generator expression or list comprehension to ensure a new empty list is created for each element in list l:
collections.OrderedDict(zip(l, ([] for _ in l))
using the generator expression ([] for _ in l) creates a new empty list for every element in l instead of copying the reference to a single empty list. The easiest way to check this is to use the id function to check the underlying ids of the objects. Here we'll compare your original method to the new method:
# The ids come out the same, indicating that the objects are reference to the same underlying list
>>> [id(x) for x in [[]] * len(l)]
[2746221080960, 2746221080960]
# The ids come out different, indicating that they point to different underlying lists
>>> [id(x) for x in ([] for _ in l)]
[2746259049600, 2746259213760]