Replace Spaces With Underscores in Python

In this guide, we will explore various methods for replacing spaces with underscores, but you can apply these techniques to substitute any character with another of your choosing.

Replace spaces with underscores using the replace() method

To replace spaces with underscores using the replace() method in Python:

1. Call the replace() method on the string you want to modify.
2. Pass two arguments to the replace() method: the first is the substring we want to replace (a space character " ") and the second is the string we want to replace it with (an underscore "_").
3. The replace() method returns a new string with all the occurrences of spaces replaced with underscores.

Here, is an example:

my_str = "python is awesome"

new_str = my_str.replace(" ", "_")

print(new_str)  # Output: python_is_awesome

The replace() method is used to perform a simple replacement of a substring with another string.

However, if we need to search for a complex pattern and replace it with a string, we should use the re.sub() method from the re module. This method allows us to use regular expressions to match patterns in the string and replace them with another string.

Replace whitespaces with underscores using the re.sub() method

When working with strings in Python, it may sometimes be necessary to replace all whitespace characters (such as spaces, tabs, carriage returns, line feeds, or form feeds) with another character. This can be achieved using the re.sub() method from the re module.

In this guide, we will replace all whitespaces with underscores using the re.sub() method.

To replace spaces with underscores using the re.sub() method:

1. First, we have to import the regular expression re module.
2. Use re.sub() to replace all whitespaces with underscores in a string. The first argument is the pattern to search for ("\s"), the second is the string we want to replace it with ("_"), and the third is the string to perform the replacement on.
3. The re.sub() method returns a new string with all the occurrences of spaces replaced with underscores.

Here, is an example:

import re

my_str = "python\tis awesome"

new_str = re.sub("\s", "_", my_str)

print(new_str)  # Output: python_is_awesome

The \s matches all whitespace characters like space, tab, carriage return, line feed, or form feed.

We can also pass a string with the space character  " " in the re.sub() method if we only want to replace the space character.

String with multiple spaces between words

If a string contains multiple spaces between words and you want to replace them with a single underscore or another character, you can use the re.sub() method from the re module.

To do this, we have to pass a regular expression that matches one or more space characters (" +") as the search value and an underscore ("_") as the replacement value.

Here, is an example:

import re

my_str = "python   is awesome"

new_str = re.sub(" +", "_", my_str)
print(new_str)  # Output: python_is_awesome

new_string_two = re.sub(" ", "_", my_str)
print(new_string_two)  # Output: python___is_awesome

The + character in a regular expression matches the previous character one or more times.

E.g.

"e+" matches "ee" and "e" in the string "breeze".

 Whereas,  "e" matches 'e', 'e', and 'e' in the string "breeze".

In our example, the regex pattern " +" matches one or more spaces. The re.sub() method searches for all the occurrences of one or more spaces in the string my_str and replaces them with an underscore.

Note: Notice that there is a space character before the + character.

String with leading and trailing spaces

To replace spaces with underscores in a string that has leading and trailing spaces, we first need to use the trim() method to remove them. Then, we can use either the replace() or re.sub() method to replace spaces with underscores.

For example:

my_str = " python is awesome "

stripped_str = my_str.strip()

new_str = stripped_str.replace(" ", "_")

print(new_str)  # Output: python_is_awesome

Replace spaces with underscores using the split() and join() method

To replace spaces with underscores using the split() and join() method:

1. First, we split the string into a list of substrings at each occurrence of space using the split() method.
2. Second, we join the list of substrings using the join() method with an underscore as a separator.

Here is an example:

my_str = "python is awesome"

list_str = my_str.split()

new_str = "_".join(list_str)

print(new_str)  # python is awesome

Note: The split() method also takes a separator. The default separator is any whitespace.

Replace spaces with underscores using list comprehension

Many Python developers find list comprehension to be a powerful and concise tool. If you’re familiar with this technique, you can use it to replace spaces with underscores in a string.

Here, is an example:

my_str = "python is awesome"

list_str = ["_" if x == " " else x for x in my_str]

print(list_str)

# Output: ['p', 'y', 't', 'h', 'o', 'n', '_', 'i', 's', '_', 'a', 'w', 'e', 's', 'o', 'm', 'e']

new_str = "".join(list_str)

print(new_str)  # Output: python_is_awesome

First, we use list comprehension to create a list list_str. The logic is simple: if a character is a space, we replace it with an underscore; otherwise, we keep the character unchanged.

Second, we use the join() method to combine all the elements of the list list_str into a single string. We pass an empty string as the delimiter to join the elements without any separation.

Replace spaces with underscores using the for loop

We can also use the good old for loop to replace spaces with underscores.

For example:

my_str = "python is awesome"

new_str = ""
for char in my_str:
    if char == " ":
        new_str += "_"
    else:
        new_str += char

print(new_str)  # Ouput: python_is_awesome

In the above example:

1. We used for loop to iterate over each character of a string my_str.
2. If the character is a space, we append an underscore to a new string new_str. Otherwise, we append the character itself to the new string new_str.
3. The result is a string new_str with all spaces replaced with underscores. 

Replace spaces with underscores using the while loop

We can also use a while loop to replace spaces with underscores.

Here, is an example:

my_str = "python is awesome"

index = 0
new_str = ""

while index < len(my_str):
    if my_str[index] == " ":
        new_str += "_"
    else:
        new_str += my_str[index]
    index += 1

print(new_str)

In the above example:

1. We initialize the index to 0 and new_str to an empty string.
2. The while loop runs as long as the value of the index is less than the length of the string my_str.
3. Inside the loop, we use an if statement to check if the current character is a space " ". If it is a space, we add an underscore "_" to string new_str. Otherwise, we add the current character to string new_str.
4. The result is a string new_str with all spaces replaced with underscores.