Appending DataFrame to empty DataFrame in {Key: Empty DataFrame (with columns)} - python-3.x

Question

I am struggling to understand this one.
I have a regular df (same columns as the empty df in dict) and an empty df which is a value in a dictionary (the keys in the dict are variable based on certain inputs, so can be just one key/value pair or multiple key/value pairs - think this might be relevant). The dict structure is essentially:
{key: [[Empty DataFrame
Columns: [list of columns]
Index: []]]}
I am using the following code to try and add the data:
dict[key].append(df, ignore_index=True)
The error I get is:
temp_dict[product_match].append(regular_df, ignore_index=True)
TypeError: append() takes no keyword arguments
Is this error due to me mis-specifying the value I am attempting to append the df to (like am I trying to append the df to the key instead here) or something else?

Answer 1

Your dictionary contains a list of lists at the key, we can see this in the shown output:
{key: [[Empty DataFrame Columns: [list of columns] Index: []]]}
# ^^ list starts ^^ list ends
For this reason dict[key].append is calling list.append as mentioned by #nandoquintana.
To append to the DataFrame access the specific element in the list:
temp_dict[product_match][0][0].append(df, ignore_index=True)
Notice there is no inplace version of append. append always produces a new DataFrame:
Sample Program:
import numpy as np
import pandas as pd
temp_dict = {
'key': [[pd.DataFrame()]]
}
product_match = 'key'
np.random.seed(5)
df = pd.DataFrame(np.random.randint(0, 100, (5, 4)))
temp_dict[product_match][0][0].append(df, ignore_index=True)
print(temp_dict)
Output (temp_dict was not updated):
{'key': [[Empty DataFrame
Columns: []
Index: []]]}
The new DataFrame will need to be assigned to the correct location.
Either a new variable:
some_new_variable = temp_dict[product_match][0][0].append(df, ignore_index=True)
some_new_variable
0 1 2 3
0 99 78 61 16
1 73 8 62 27
2 30 80 7 76
3 15 53 80 27
4 44 77 75 65
Or back to the list:
temp_dict[product_match][0][0] = (
temp_dict[product_match][0][0].append(df, ignore_index=True)
)
temp_dict
{'key': [[ 0 1 2 3
0 99 78 61 16
1 73 8 62 27
2 30 80 7 76
3 15 53 80 27
4 44 77 75 65]]}
Assuming there the DataFrame is actually an empty DataFrame, append is unnecessary as simply updating the value at the key to be that DataFrame works:
temp_dict[product_match] = df
temp_dict
{'key': 0 1 2 3
0 99 78 61 16
1 73 8 62 27
2 30 80 7 76
3 15 53 80 27
4 44 77 75 65}
Or if list of list is needed:
temp_dict[product_match] = [[df]]
temp_dict
{'key': [[ 0 1 2 3
0 99 78 61 16
1 73 8 62 27
2 30 80 7 76
3 15 53 80 27
4 44 77 75 65]]}

Answer 2

Maybe you have an empty list at dict[key]?
Remember that "append" list method (unlike Pandas dataframe one) only receives one parameter:
https://docs.python.org/3/tutorial/datastructures.html#more-on-lists
https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.append.html