python-files-notes.md

Python Files Notes

Say you have the following file number-words.txt with the contents:

one
two
three

Reading a file

One line at a time

To open a file for reading, you use the open method, passing in the r option for "read".

file = open('number-words.txt', 'r')
for line in file:
    print(line.strip())
file.close()

Reading whole file in as a list of lines

With readlines, you read in the contents of a file as a list of lines:

file = open('number-words.txt', 'r')
number_words_lines = file.readlines() 
file.close()

This will give you a newline character at the end of each line in your list, so it's common to use a list comprehension to process the lines:

number_words = [elt.strip() for elt in file.readlines()]

Reading whole file in as a string

You can read the entire contents in at once with read.

file = open('number-words.txt', 'r')
number_words_contents = file.read()
file.close()

Remember to close the file handle when you are finished.

Writing to a file

Writing to a file a line at a time

To write to a file, you need to open it with the w flag:

file = open('number-words.txt', 'w')
file.write('four')
file.close()

HOWEVER: This will not append the word "four" to the file, it will overwrite it with the word "four".

To append, use the a flag:

file = open('number-words.txt', 'a')
file.write('four')
file.close()

Writing a list to a file all at once

You can use writelines for this:

new_numbers = ['four', 'five']
with open('number-words.txt', 'a') as file:
    file.writelines(new_numbers)

Auto closing a file with with

Note: the current convention is to do it this way most of the time.

The advantage of using with is that it closes the file handle automatically:

with open('number-words.txt', 'r') as file:
    for line in file:
        print(line.strip())

Working with csv files

Reading a csv file

Say you have a csv file users.csv:

customerNo,firstName,lastName,birthDate,mailingAddress
1,John,Dunbar,13/06/1945,"1600 Amphitheatre Parkway Mountain View, CA 94043 United States"
2,Bob,Down,25/02/1919,"1601 Willow Rd. Menlo Park, CA 94025 United States"
3,Alice,Wunderland,08/08/1985,"One Microsoft Way Redmond, WA 98052-6399 United States"
4,Bill,Jobs,10/07/1973,"2701 San Tomas Expressway Santa Clara, CA 95050 United States"

You can read and work with this with the csv module:

import csv

names = []

with open('users.csv', 'r') as f:
    rows = csv.reader(f)
    headers = next(rows)
    for row in rows:
        names.append("{} {}".format(row[1], row[2]))

print(names)
# => ['John Dunbar', 'Bob Down', 'Alice Wunderland', 'Bill Jobs']

Writing a csv file

You can write with a csv writer:

import csv 

with open('users.csv', 'a') as file:
    writer = csv.writer(file)
    writer.writerow([5, 'John', 'Doe', '123 Any St. San Francisco, CA 99999'])

The writer method also takes "dialect arguments" so you can change the writing format. For example, you might want to change to tab delimited instead of commas. More documentation on dialects here