14_Reading_and_Writing_Files.md

Python Cheat Sheet

Basic cheatsheet for Python mostly based on the book written by Al Sweigart, Automate the Boring Stuff with Python under the Creative Commons license and many other sources.

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Reading and Writing Files

The File Reading/Writing Process

To read/write to a file in Python, you will want to use the with statement, which will close the file for you after you are done.

Opening and reading files with the open function

with open('C:\\Users\\your_home_folder\\hello.txt') as hello_file:
    hello_content = hello_file.read()

hello_content

Alternatively, you can use the readlines() method to get a list of string values from the file, one string for each line of text:

with open('sonnet29.txt') as sonnet_file:
    sonnet_file.readlines()

You can also iterate through the file line by line:

with open('sonnet29.txt') as sonnet_file:
    for line in sonnet_file: # note the new line character will be included in the line
        print(line, end='')

Writing to Files

with open('bacon.txt', 'w') as bacon_file:
    bacon_file.write('Hello world!\n')

with open('bacon.txt', 'a') as bacon_file:
    bacon_file.write('Bacon is not a vegetable.')

with open('bacon.txt') as bacon_file:
    content = bacon_file.read()

print(content)

Saving Variables with the shelve Module

To save variables:

import shelve

cats = ['Zophie', 'Pooka', 'Simon']
with shelve.open('mydata') as shelf_file:
    shelf_file['cats'] = cats

To open and read variables:

with shelve.open('mydata') as shelf_file:
    print(type(shelf_file))
    print(shelf_file['cats'])

Just like dictionaries, shelf values have keys() and values() methods that will return list-like values of the keys and values in the shelf. Since these methods return list-like values instead of true lists, you should pass them to the list() function to get them in list form.

with shelve.open('mydata') as shelf_file:
    print(list(shelf_file.keys()))
    print(list(shelf_file.values()))

Saving Variables with pprint.pformat

import pprint

cats = [{'name': 'Zophie', 'desc': 'chubby'}, {'name': 'Pooka', 'desc': 'fluffy'}]
pprint.pformat(cats)

with open('myCats.py', 'w') as file_obj:
    file_obj.write('cats = {}\n'.format(pprint.pformat(cats)))

Reading ZIP Files

import zipfile, os

os.chdir('C:\\')    # move to the folder with example.zip
with zipfile.ZipFile('example.zip') as example_zip:
    print(example_zip.namelist())
    spam_info = example_zip.getinfo('spam.txt')
    print(spam_info.file_size)
    print(spam_info.compress_size)
    print('Compressed file is %sx smaller!' % (round(spam_info.file_size / spam_info.compress_size, 2)))

Extracting from ZIP Files

The extractall() method for ZipFile objects extracts all the files and folders from a ZIP file into the current working directory.

import zipfile, os

os.chdir('C:\\')    # move to the folder with example.zip

with zipfile.ZipFile('example.zip') as example_zip:
    example_zip.extractall()

The extract() method for ZipFile objects will extract a single file from the ZIP file. Continue the interactive shell example:

with zipfile.ZipFile('example.zip') as example_zip:
    print(example_zip.extract('spam.txt'))
    print(example_zip.extract('spam.txt', 'C:\\some\\new\\folders'))

Creating and Adding to ZIP Files

import zipfile

with zipfile.ZipFile('new.zip', 'w') as new_zip:
    new_zip.write('spam.txt', compress_type=zipfile.ZIP_DEFLATED)

This code will create a new ZIP file named new.zip that has the compressed contents of spam.txt.