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Writing CSV files in Python

Writing CSV files in Python

In this tutorial, we will learn to write CSV files with different formats in Python with the help of examples.

We are going to exclusively use the csv module built into Python for this task. But first, we will have to import the module as :

import csv

We have already covered the basics of how to use the csv module to read and write into CSV files. If you don't have any idea on using the csv module, check out our tutorial on Python CSV: Read and Write CSV files


Basic Usage of csv.writer()

Let's look at a basic example of using csv.writer() to refresh your existing knowledge.

Example 1: Write into CSV files with csv.writer()

Suppose we want to write a CSV file with the following entries:

SN,Name,Contribution
1,Linus Torvalds,Linux Kernel
2,Tim Berners-Lee,World Wide Web
3,Guido van Rossum,Python Programming

Here's how we do it.

import csv
with open('innovators.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(["SN", "Name", "Contribution"])
    writer.writerow([1, "Linus Torvalds", "Linux Kernel"])
    writer.writerow([2, "Tim Berners-Lee", "World Wide Web"])
    writer.writerow([3, "Guido van Rossum", "Python Programming"])

When we run the above program, an innovators.csv file is created in the current working directory with the given entries.

Here, we have opened the innovators.csv file in writing mode using open() function.

To learn more about opening files in Python, visit: Python File Input/Output

Next, the csv.writer() function is used to create a writer object. The writer.writerow() function is then used to write single rows to the CSV file.


Example 2: Writing Multiple Rows with writerows()

If we need to write the contents of the 2-dimensional list to a CSV file, here's how we can do it.

import csv
row_list = [["SN", "Name", "Contribution"],
             [1, "Linus Torvalds", "Linux Kernel"],
             [2, "Tim Berners-Lee", "World Wide Web"],
             [3, "Guido van Rossum", "Python Programming"]]
with open('protagonist.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerows(row_list)

The output of the program is the same as in Example 1.

Here, our 2-dimensional list is passed to the writer.writerows() function to write the content of the list to the CSV file.


Now let's see how we can write CSV files in different formats. We will then learn how to customize the csv.writer() function to write them.


CSV Files with Custom Delimiters

By default, a comma is used as a delimiter in a CSV file. However, some CSV files can use delimiters other than a comma. Few popular ones are | and \t.

Suppose we want to use | as a delimiter in the innovators.csv file of Example 1. To write this file, we can pass an additional delimiter parameter to the csv.writer() function.

Let's take an example.

Example 3: Write CSV File Having Pipe Delimiter

import csv
data_list = [["SN", "Name", "Contribution"],
             [1, "Linus Torvalds", "Linux Kernel"],
             [2, "Tim Berners-Lee", "World Wide Web"],
             [3, "Guido van Rossum", "Python Programming"]]
with open('innovators.csv', 'w', newline='') as file:
    writer = csv.writer(file, delimiter='|')
    writer.writerows(data_list)

Output

SN|Name|Contribution
1|Linus Torvalds|Linux Kernel
2|Tim Berners-Lee|World Wide Web
3|Guido van Rossum|Python Programming

As we can see, the optional parameter delimiter = '|' helps specify the writer object that the CSV file should have | as a delimiter.


CSV files with Quotes

Some CSV files have quotes around each or some of the entries.

Let's take quotes.csv as an example, with the following entries:

"SN";"Name";"Quotes"
1;"Buddha";"What we think we become"
2;"Mark Twain";"Never regret anything that made you smile"
3;"Oscar Wilde";"Be yourself everyone else is already taken"

Using csv.writer() by default will not add these quotes to the entries.

In order to add them, we will have to use another optional parameter called quoting.

Let's take an example of how quoting can be used around the non-numeric values and ; as delimiters.

Example 4: Write CSV files with quotes

import csv
row_list = [
    ["SN", "Name", "Quotes"],
    [1, "Buddha", "What we think we become"],
    [2, "Mark Twain", "Never regret anything that made you smile"],
    [3, "Oscar Wilde", "Be yourself everyone else is already taken"]
]
with open('quotes.csv', 'w', newline='') as file:
    writer = csv.writer(file, quoting=csv.QUOTE_NONNUMERIC, delimiter=';')
    writer.writerows(row_list)

Output

"SN";"Name";"Quotes"
1;"Buddha";"What we think we become"
2;"Mark Twain";"Never regret anything that made you smile"
3;"Oscar Wilde";"Be yourself everyone else is already taken"

Here, the quotes.csv file is created in the working directory with the above entries.

As you can see, we have passed csv.QUOTE_NONNUMERIC to the quoting parameter. It is a constant defined by the csv module.

csv.QUOTE_NONNUMERIC specifies the writer object that quotes should be added around the non-numeric entries.

There are 3 other predefined constants you can pass to the quoting parameter:

  • csv.QUOTE_ALL - Specifies the writer object to write CSV file with quotes around all the entries.
  • csv.QUOTE_MINIMAL - Specifies the writer object to only quote those fields which contain special characters (delimiter, quotechar or any characters in lineterminator)
  • csv.QUOTE_NONE - Specifies the writer object that none of the entries should be quoted. It is the default value.

CSV files with custom quoting character

We can also write CSV files with custom quoting characters. For that, we will have to use an optional parameter called quotechar.

Let's take an example of writing quotes.csv file in Example 4, but with * as the quoting character.

Example 5: Writing CSV files with custom quoting character

import csv
row_list = [
    ["SN", "Name", "Quotes"],
    [1, "Buddha", "What we think we become"],
    [2, "Mark Twain", "Never regret anything that made you smile"],
    [3, "Oscar Wilde", "Be yourself everyone else is already taken"]
]
with open('quotes.csv', 'w', newline='') as file:
    writer = csv.writer(file, quoting=csv.QUOTE_NONNUMERIC,
                        delimiter=';', quotechar='*')
    writer.writerows(row_list)

Output

*SN*;*Name*;*Quotes*
1;*Buddha*;*What we think we become*
2;*Mark Twain*;*Never regret anything that made you smile*
3;*Oscar Wilde*;*Be yourself everyone else is already taken*

Here, we can see that quotechar='*' parameter instructs the writer object to use * as quote for all non-numeric values.


Dialects in CSV module

Notice in Example 5 that we have passed multiple parameters (quoting, delimiter and quotechar) to the csv.writer() function.

This practice is acceptable when dealing with one or two files. But it will make the code more redundant and ugly once we start working with multiple CSV files with similar formats.

As a solution to this, the csv module offers dialect as an optional parameter.


Dialect helps in grouping together many specific formatting patterns like delimiter, skipinitialspace, quoting, escapechar into a single dialect name.

It can then be passed as a parameter to multiple writer or reader instances.


Example 6: Write CSV file using dialect

Suppose we want to write a CSV file (office.csv) with the following content:

"ID"|"Name"|"Email"
"A878"|"Alfonso K. Hamby"|"alfonsokhamby@rhyta.com"
"F854"|"Susanne Briard"|"susannebriard@armyspy.com"
"E833"|"Katja Mauer"|"kmauer@jadoop.com"

The CSV file has quotes around each entry and uses | as a delimiter.

Instead of passing two individual formatting patterns, let's look at how to use dialects to write this file.

import csv
row_list = [
    ["ID", "Name", "Email"],
    ["A878", "Alfonso K. Hamby", "alfonsokhamby@rhyta.com"],
    ["F854", "Susanne Briard", "susannebriard@armyspy.com"],
    ["E833", "Katja Mauer", "kmauer@jadoop.com"]
]
csv.register_dialect('myDialect',
                     delimiter='|',
                     quoting=csv.QUOTE_ALL)
with open('office.csv', 'w', newline='') as file:
    writer = csv.writer(file, dialect='myDialect')
    writer.writerows(row_list)

Output

"ID"|"Name"|"Email"
"A878"|"Alfonso K. Hamby"|"alfonsokhamby@rhyta.com"
"F854"|"Susanne Briard"|"susannebriard@armyspy.com"
"E833"|"Katja Mauer"|"kmauer@jadoop.com"

Here, office.csv is created in the working directory with the above contents.

From this example, we can see that the csv.register_dialect() function is used to define a custom dialect. Its syntax is:

csv.register_dialect(name[, dialect[, **fmtparams]])

The custom dialect requires a name in the form of a string. Other specifications can be done either by passing a sub-class of the Dialect class, or by individual formatting patterns as shown in the example.


While creating the writer object, we pass dialect='myDialect' to specify that the writer instance must use that particular dialect.

The advantage of using dialect is that it makes the program more modular. Notice that we can reuse myDialect to write other CSV files without having to re-specify the CSV format.


Write CSV files with csv.DictWriter()

The objects of csv.DictWriter() class can be used to write to a CSV file from a Python dictionary.

The minimal syntax of the csv.DictWriter() class is:

csv.DictWriter(file, fieldnames)

Here,

  • file - CSV file where we want to write to
  • fieldnames - a list object which should contain the column headers specifying the order in which data should be written in the CSV file

Example 7: Python csv.DictWriter()

import csv

with open('players.csv', 'w', newline='') as file:
    fieldnames = ['player_name', 'fide_rating']
    writer = csv.DictWriter(file, fieldnames=fieldnames)

    writer.writeheader()
    writer.writerow({'player_name': 'Magnus Carlsen', 'fide_rating': 2870})
    writer.writerow({'player_name': 'Fabiano Caruana', 'fide_rating': 2822})
    writer.writerow({'player_name': 'Ding Liren', 'fide_rating': 2801})

Output

The program creates a players.csv file with the following entries:

player_name,fide_rating
Magnus Carlsen,2870
Fabiano Caruana,2822
Ding Liren,2801

The full syntax of the csv.DictWriter() class is:

csv.DictWriter(f, fieldnames, restval='', extrasaction='raise', dialect='excel', *args, **kwds)

To learn more about it in detail, visit: Python csv.DictWriter() class


CSV files with lineterminator

A lineterminator is a string used to terminate lines produced by writer objects. The default value is \r\n. You can change its value by passing any string as a lineterminator parameter.

However, the reader object only recognizes \n or \r as lineterminator values. So using other characters as line terminators is highly discouraged.


doublequote & escapechar in CSV module

In order to separate delimiter characters in the entries, the csv module by default quotes the entries using quotation marks.

So, if you had an entry: He is a strong, healthy man, it will be written as: "He is a strong, healthy man".

Similarly, the csv module uses double quotes in order to escape the quote character present in the entries by default.

If you had an entry: Go to "programiz.com", it would be written as: "Go to ""programiz.com""".

Here, we can see that each " is followed by a " to escape the previous one.


doublequote

It handles how quotechar present in the entry themselves are quoted. When True, the quoting character is doubled and when False, the escapechar is used as a prefix to the quotechar. By default its value is True.

escapechar

escapechar parameter is a string to escape the delimiter if quoting is set to csv.QUOTE_NONE and quotechar if doublequote is False. Its default value is None.

Example 8: Using escapechar in csv writer

import csv
row_list = [
    ['Book', 'Quote'],
    ['Lord of the Rings',
        '"All we have to decide is what to do with the time that is given us."'],
    ['Harry Potter', '"It matters not what someone is born, but what they grow to be."']
]
with open('book.csv', 'w', newline='') as file:
    writer = csv.writer(file, escapechar='/', quoting=csv.QUOTE_NONE)
    writer.writerows(row_list)

Output

Book,Quote
Lord of the Rings,/"All we have to decide is what to do with the time that is given us./"
Harry Potter,/"It matters not what someone is born/, but what they grow to be./"

Here, we can see that / is prefix to all the " and , because we specified quoting=csv.QUOTE_NONE.

If it wasn't defined, then, the output would be:

Book,Quote
Lord of the Rings,"""All we have to decide is what to do with the time that is given us."""
Harry Potter,"""It matters not what someone is born, but what they grow to be."""

Since we allow quoting, the entries with special characters(" in this case) are double-quoted. The entries with delimiter are also enclosed within quote characters.(Starting and closing quote characters)

The remaining quote characters are to escape the actual " present as part of the string, so that they are not interpreted as quotechar.


Note: The csv module can also be used for other file extensions (like: .txt) as long as their contents are in proper structure.

Recommended Reading: Read CSV Files in Python