How to Convert Between Comma-Separated Strings and Lists in Python

This guide covers two common and related tasks:

1. Converting a comma-separated string (or a string with other separators) into a list.
2. Converting a list (of strings or numbers) into a comma-separated string.

We'll explore the most efficient and Pythonic methods, using str.split(), str.join(), list comprehensions, and map().

Converting a Comma-Separated String to a List

Basic Splitting with str.split()

The str.split() method is the primary tool for converting a delimited string into a list:

my_str = 'tutorial,reference,com'
my_list = my_str.split(',')     # Split on commas
print(my_list)                  # Output: ['tutorial', 'reference', 'com']

• The my_str.split(',') splits the string by commas and constructs a list of substrings.

Handling Different Separators

str.split() can handle any separator string, not just commas:

my_str = 'tutorial reference com'
my_list = my_str.split()        # Split on any whitespace, by not providing an argument
print(my_list)                  # Output: ['tutorial', 'reference', 'com']

my_str = 'tutorial, reference, com'
my_list = my_str.split(', ')    # Split on comma and space
print(my_list)                  # Output: ['tutorial', 'reference', 'com']

• The split() method takes an optional separator parameter. If no value is provided, it defaults to splitting on whitespace.

Handling Leading/Trailing Separators

If your string starts or ends with the delimiter, split() will create empty strings in the list. To remove those:

my_str = ',tutorial,reference,com,'
my_list = [item for item in my_str.split(',') if item]
print(my_list) # Output: ['tutorial', 'reference', 'com']

• The if item condition in the list comprehension filters out empty strings.
• Another, less readable way, to do this is to use filter: my_list = list(filter(None, my_str.split(',')))

Converting to a List of Integers

If your string contains numbers, convert them to integers after splitting:

my_str = '1,2,3,4,5'
my_list = [int(item) for item in my_str.split(',')]
print(my_list)  # Output: [1, 2, 3, 4, 5]

#OR
my_list = list(map(int, my_str.split(',')))
print(my_list)  # Output: [1, 2, 3, 4, 5]

• List Comprehension (recommended): [int(item) for item in ...] converts each substring to an integer.
• map(): map(int, ...) applies the int() function to each element. This returns a map object, which is then converted to a list using list(). The list comprehension is generally preferred for readability.
• If your string also contains other characters, and you only want to keep the numbers, you can check with isdigit() and int() to convert the string to a number.

Converting a List to a Comma-Separated String

Joining a List of Strings

The str.join() method is the most efficient and Pythonic way to join a list of strings into a single string:

list_of_strings = ['tutorial', 'reference', 'com']

my_str = ','.join(list_of_strings)  # Join with commas
print(my_str)           # Output: tutorial,reference,com

my_str_space = ' '.join(list_of_strings) # Join with space
print(my_str_space)     # Output: tutorial reference com

• The method takes a list of strings as an argument, and joins them using a separator.

Joining a List of Integers

If your list contains numbers, you first need to convert them to strings:

list_of_integers = [1, 3, 5, 7]

# Using a generator expression (more efficient)
my_str = ','.join(str(item) for item in list_of_integers)
print(my_str)  # Output: 1,3,5,7

# OR, using map (less readable, but also works):
my_str = ','.join(map(str, list_of_integers))
print(my_str)  # Output: 1,3,5,7

• Generator Expression: (str(item) for item in list_of_integers) converts each integer to a string as it's being used by join(). This is memory-efficient.
• map(str, list_of_integers): Applies str() to each integer, creating a map object (an iterator). This is less readable than the generator expression.