How to get the average length of the strings in each column in csv?

You can try pandas. In case you don't have pandas installed, do pip install pandas to install pandas.

import pandas as pd
# df = pd.read_csv('my_csv.csv')
df = pd.DataFrame([['aa', 'aaa', 'a'], ['bbbb', 'bbbbbb', 'aa']], 
                  columns=['Header1', 'Header2', 'Header3'])
result = pd.DataFrame([[]])
for col in df:
    result[col] = df[col].apply(len).mean()

result.to_csv('result.csv')

Hope this helps!

You can zip the rows and map the columns to len and use statistics.mean to calculate averages:

import csv
from statistics import mean
with open('someFile.csv', 'r', newline='') as f, open('results.csv', 'w', newline='') as output:
    reader = csv.reader(f, delimiter=' ', skipinitialspace=True)
    headers = next(reader)
    writer = csv.writer(output, delimiter = ' ')
    writer.writerow(headers)
    writer.writerow([mean(map(len, col)) for col in zip(*reader)])

Here is a simple code. I provided two blocks, if no null in dataframe and if Nulls are present.

import pandas as pd

#df = pd.DataFrame([['aa','aaa','a'],['bbbb','bbbbbb','aa']],columns=['Header1','Header2','Header3'])
df = pd.read_csv('file.csv')

#if No Null
No_of_Row=df.shape[0]
for Col in df.sum():
    print(len(Col)/No_of_Row)

#if Null are there
for Col,Header in zip(df.sum(),df.columns):
    print(len(Col)/df[[Header]].dropna().shape[0])

This isn't the best way to do it. There are other ways to do this quickly. However, I do think this is a fairly straight forward and easy-to-understand example that was put together very hastily. I used this on your example and it works.

import csv

# replace "yourusername" with your PC user name
input_file = 'C:/Users/yourusername/Desktop/someFile.csv' 
output_file = 'C:/Users/yourusername/Desktop/output.csv'

csv_file = open(input_file, newline='')  # opening csv file
info = list(csv.reader(csv_file))  # convert data in csv file to array/list
csv_file.close()

length = len(info[0])  # if you ever add more headers, this will account for it
avg_container = [0 for i in range(length)]  # creates empty array with zeros for each header
n = len(info[1:])  # for dividing by n to get average

# adding the lengths of all the items to one sum for each "column"
for k in info[1:]:
    for n,i in enumerate(k):
        avg_container[n] += len(i)

# diviving all sums by n
for i in range(len(avg_container)):
    avg_container[i] = avg_container[i]/n

# combine header and average array into one item to write to csv
avg_output = []
avg_output.extend((info[0],avg_container))
print(avg_output)  # just for you to see for yourself

# outputting the new file
output_csv = open(output_file, 'w', newline='')  # creates an instance of the file
csv_writer = csv.writer(output_csv)  # creates an "Writer" to write to the csv
csv_writer.writerows(avg_output)  # outputs the avg_output variable to the csv file
output_csv.close()  # finished

References

How to import a csv-file into a data array?

Create a .csv file with values from a Python list

Writing a Python list of lists to a csv file