File I/o

Objectives

  • Read text files in Python
  • Write text files in Python
  • Use "with" blocks when reading / writing files
  • Describe the different ways to open a file
  • Read CSV files in Python
  • Write CSV files in Python

Reading Files

  • You can read a file with the open function

  • open returns a file object to you

  • You can read a file object with the read method

File Read Example

story.txt
This short story is really short.
reading_files.py
file = open("story.txt")
file.read()

Cursor Movement

  • Python reads files by using a cursor
  • This is like the cursor you see when you're typing
  • After a file is read, the cursor is at the end
  • To move the cursor, use the seek method
  • To read only part of a file, pass a number of characters into read, or use readline
  • To get a list of all lines, use readlines

Closing a File

  • You can close a file with the close method
  • You can check if a file is closed with the closed attribute
  • Once closed, a file can't be read again
  • Always close files - it frees up system resources!

with Blocks 

file = open("story.txt")
file.read()
file.close()

file.closed # True

Option 1

with open("story.txt") as file:
    file.read()

file.closed # True

Option 2

Writing to Text Files

Writing Files

  • You can also use open to write to a file
  • Need to specify the "w" flag as the second argument

Writing Files Example

with open("haiku.txt", "w") as file:
    file.write("Writing files is great\n")
    file.write("Here's another line of text\n")
    file.write("Closing now, goodbye!")

IMPORTANT: If you open the file again to write, the original contents will be deleted!

Modes for Opening Files

  • r - Read a file (no writing) - this is the default
  • w - Write to a file (previous contents removed)
  • a - Append to a file (previous contents not removed)
  • r+ - Read and write to a file (writing based on cursor)

Truncating Files

file.truncate - removes all text starting from the current cursor position

YOUR TURN

Reading CSV Files

Reading CSV Files

  • CSV files are a common file format for tabular data
  • We can read CSV files just like other text files
  • Python has a built-in CSV module to read/write CSVs more easily

Reading CSV Example

fighters.csv
Name,Country,Height (in cm)
Ryu,Japan,175
Ken,USA,175
Chun-Li,China,165
Guile,USA,182
E. Honda,Japan,185
Dhalsim,India,176
Blanka,Brazil,192
Zangief,Russia,214
reading_csv.py
with open("fighters.csv") as file:
    file.read()

Don't do this!

CSV Module

  • reader - lets you iterate over rows of the CSV as lists
  • DictReader - lets you iterate over rows of the CSV as OrderedDicts
  • Keys are determined by the header row
  • An OrderedDict is like a dictionary, but it remembers the order in which keys were inserted

CSV Module Examples

from csv import reader
with open("fighters.csv") as file:
    csv_reader = reader(file)
    for row in csv_reader:
        # each row is a list!
        print(row) 
from csv import DictReader
with open("fighters.csv") as file:
    csv_reader = DictReader(file)
    for row in csv_reader:
        # each row is an OrderedDict!
        print(row)

reader

DictReader

Other Delimiters

from csv import reader
with open("example.csv") as file:
    csv_reader = reader(file, delimiter="|")
    for row in csv_reader:
        # each row is a list!
        print(row)

Readers accept a delimiter kwarg in case your data isn't separated by commas.

Writing to CSV Files

Writing CSV Files

  • writer - creates a writer object for writing to CSV
  • writerow - method on a writer to write a row to the CSV

Using lists

Writing CSV Files Example

Using lists

from csv import writer
with open("fighters.csv", "w") as file:
    csv_writer = writer(file)
    csv_writer.writerow(["Character", "Move"])
    csv_writer.writerow(["Ryu", "Hadouken"])

Writing CSV Files

Using Dictionaries

  • DictWriter - creates a writer object for writing using dictionaries
  • fieldnames - kwarg for the DictWriter specifying headers
  • writeheader - method on a writer to write header row 
  • writerow - method on a writer to write a row based on a dictionary

Writing CSV Files Example

Using Dictionaries

from csv import DictWriter
with open("example.csv", "w") as file:
    headers = ["Character", "Move"]
    csv_writer = DictWriter(file, fieldnames=headers)
    csv_writer.writeheader()
    csv_writer.writerow({
        "Character": "Ryu", 
        "Move": "Hadouken"
    })

Dictionaries or Lists?

  • Using lists often means less code
  • Using dictionaries is often more explicit

Recap

  • You can read and write text files using open and with
  • Files are automatically closed after use when using with
  • You can set the mode for open to read, write, or append
  • The CSV module lets you read / write CSV files
  • You can read / write CSVs using lists or dictionaries

YOUR TURN