Tag: write

How to Read a File in Python

You have seen various types of data holders before: integers, strings, lists. But so far, we have not discussed how to read or write files.

Read file

The Python programming language provides the ability to work with files using open().

Python programming treats some files as text files, where lines are separated by newline characters \n. You can open regular files with the paramater r.

Other files are considered binary and can be handled in a way that is similar to the C programming language. They need to be opened with the parameters rb.

read file into string

This is a sample program that shows how to read data from a file.

The file needs to be in the same directory as the program, if not you need to specify a path.

Create python script. Open editor of your choice and create new python script. Then paste the following code.

f = open("file.txt","r")
lines = f.readlines()
print(lines)

The read method readlines() reads all the contents of a file into a string.

Save the file with name example.py and run it.

read file line by line

To output line by line, you can use a for loop. The lines may include a new line character \n, that is why you can output using endl="".

f = open("filename.txt","r")
lines = f.readlines()

for line in lines:
    print(line, end="")

Another option is to remove the newline characters with the replace() method.

f = open("test.py","r")
lines = f.readlines()

for line in lines:
    line = line.replace("\n","")
    print(line)

read file with keyword

The with keyword can be used to read files too. This automatically closes your file.

#!/usr/bin/env python

# Define a filename.
filename = "bestand.py"

# Open the file as f.
# The function readlines() reads the file.
with open(filename) as f:
    content = f.readlines()

# Show the file contents line by line.
# We added the comma to print single newlines and not double newlines.
# This is because the lines contain the newline character '\n'.
for line in content:
    print(line),

The first part of the code will read the file content. All of the lines read will be stored in the variable content. The second part will iterate over every line in the variable contents.

If you do not want to read the newline characters ‘\n’, you can change the statement f.readlines() to this:

content = f.read().splitlines()

Resulting in this code:

#!/usr/bin/env python

# Define a filename.
filename = "bestand.py"

# Open the file as f.
# The function readlines() reads the file.
with open(filename) as f:
    content = f.read().splitlines()

# Show the file contents line by line.
# We added the comma to print single newlines and not double newlines.
# This is because the lines contain the newline character '\n'.
for line in content:
print(line)

While the codes above work, we should always test if the file we want to open exists. We will test first if the file does not exist, if it does it will read the file else return an error. As in the code below:

#!/usr/bin/env python
import os.path

# Define a filename.
filename = "bestand.py"

if not os.path.isfile(filename):
    print('File does not exist.')
else:
# Open the file as f.
# The function readlines() reads the file.
with open(filename) as f:
    content = f.read().splitlines()

# Show the file contents line by line.
# We added the comma to print single newlines and not double newlines.
# This is because the lines contain the newline character '\n'.
for line in content:
    print(line)

If you are new to Python programming, I highly recommend this book.

Download Python Exercises

Create and read csv

Spreadsheets often export CSV (comma seperated values) files, because they are easy to read and write. A csv file is simply consists of values, commas and newlines. While the file is called ‘comma seperate value’ file, you can use another seperator such as the pipe character.

Related course
Data Analysis with Python Pandas

Create a spreadsheet file (CSV) in Python
Let us create a file in CSV format with Python. We will use the comma character as seperator or delimter.

import csv

with open('persons.csv', 'wb') as csvfile:
    filewriter = csv.writer(csvfile, delimiter=',',
                            quotechar='|', quoting=csv.QUOTE_MINIMAL)
    filewriter.writerow(['Name', 'Profession'])
    filewriter.writerow(['Derek', 'Software Developer'])
    filewriter.writerow(['Steve', 'Software Developer'])
    filewriter.writerow(['Paul', 'Manager'])

Running this code will give us this fil persons.csv with this content:

Name,Profession
Derek,Software Developer
Steve,Software Developer
Paul,Manager

You can import the persons.csv file in your favorite office program.

Spreadsheet file created in Python

Read a spreadsheet file (csv)
If you created a csv file, we can read files row by row with the code below:

import csv

# open file
with open('persons.csv', 'rb') as f:
    reader = csv.reader(f)

    # read file row by row
    for row in reader:
        print row

This will simply show every row as a list:

['Name', 'Profession']
['Derek', 'Software Developer']
['Steve', 'Software Developer']
['Paul', 'Manager']

Perhaps you want to store that into Python lists. We get the data from the csv file and then store it into Python lists. We skip the header with an if statement because it does not belong in the lists. Full code:

import csv

# create list holders for our data.
names = []
jobs = []

# open file
with open('persons.csv', 'rb') as f:
    reader = csv.reader(f)

    # read file row by row
    rowNr = 0
    for row in reader:
        # Skip the header row.
        if rowNr >= 1:
            names.append(row[0])
            jobs.append(row[1])

        # Increase the row number
        rowNr = rowNr + 1

# Print data 
print names
print jobs

Result:

['Derek', 'Steve', 'Paul']
['Software Developer', 'Software Developer', 'Manager']

Most spreadsheet or office programs can export csv files, so we recommend you to create any type of csv file and play around with it :-)

Related course
Data Analysis with Python Pandas