How To Append Dataframe To An Empty Dataframe Using Concurrent

I want to run a function using concurrent in Python. This is the function that I have : import concurrent.futures import pandas as pd import time def putIndf(file): listSel =

Solution 1:

Essentially you are re-assigning df with each iteration and never growing it. What you probably meant (ill-advised) is to initialize an empty df and append iteratively:

df = pd.DataFrame()
...
df = df.append(file, ignore_index=True)

Nonetheless, the preferred method is to build a collection of data frames to be appended all together once outside a loop and avoid growing any complex objects like data frames inside loop.

def main():
    with concurrent.futures.ProcessPoolExecutor(max_workers=30) as executor:
        # LIST COMPREHENSION
        df_list = [file for i,file in zip(fileList, executor.map(dp.putIndf, fileList))]

        # DICTIONARY COMPREHENSION
        # df_dict = {i:file for i,file in zip(fileList, executor.map(dp.putIndf, fileList))}

    df = pd.concat(df_list, ignore_index=True)
    return df

Alternatively due to your pool process, append data frames to a list, still concatenating once outside the loop:

def main():
    df_list = []      # df_dict = {}
    with concurrent.futures.ProcessPoolExecutor(max_workers=30) as executor:
        for i,file in zip(fileList, executor.map(dp.putIndf, fileList)):
            df_list.append(file)
            # df_dict[i] = file

    df = pd.concat(df_list, ignore_index=True)
    return df