Write down a python function to convert camel case to snake case?

Camel case and snake case are ways of writing words together when we are programming.

In camel case, we write words together without spaces and we start each new word with a capital letter except for the first word. For example, if we want to write a variable name for someone's date of birth, we can write it like this: dateOfBirth.

In snake case, we write words together with an underscore symbol between them, and all the letters are lowercase. For example, if we want to write a variable name for someone's home address, we can write it like this: home_address.

Both ways are useful because they make variable names easy to read and understand. It is important to choose one and use it consistently in our code to avoid confusion.

Using a for loop with enumerate() Function

We can loop through each character in the string and check its position using the enumerate() function. This helps us to identify the uppercase letters and add underscores before them.

Example

In this example, the function camel_to_snake takes a camel case string as input and returns a snake case string. The function loops through each character in the string and adds it to the new string, adding an underscore before each uppercase letter (except the first one) to create the snake case string -

def camel_to_snake(camel_case_string):
   snake_case_string = ""
   for i, c in enumerate(camel_case_string):
      if i == 0:
         snake_case_string += c.lower()
      elif c.isupper():
         snake_case_string += "_" + c.lower()
      else:
         snake_case_string += c

   return snake_case_string

camel_case_string = "camelCaseString"
snake_case_string = camel_to_snake(camel_case_string)

print("Camel case string:", camel_case_string)
print("Snake case string:", snake_case_string)

We get the output as shown below -

Camel case string: camelCaseString
Snake case string: camel_case_string

Using regular expression with re.sub() Function

The re.sub() function is used to search for capital letters and replace them with an underscore followed by the lowercase version of that letter.

Example

In this example, the function camel_to_snake uses the re.sub() function to replace any uppercase letter with an underscore followed by the same letter in lowercase. The lower() function is then called on the resulting string to convert it to lowercase -

import re
def camel_to_snake(camel_case_string):
   snake_case_string = re.sub('([A-Z])', r'_\1', camel_case_string).lower()
   return snake_case_string

camel_case_string = "camelCaseString"
snake_case_string = camel_to_snake(camel_case_string)

print("Camel case string:", camel_case_string)
print("Snake case string:", snake_case_string)

The result produced is as shown below -

Camel case string: camelCaseString
Snake case string: camel_case_string

Using reduce() Function from functools

With the help of reduce() function, we can process the string one character at a time and build a new string while adding underscores before uppercase letters.

Example

In this example, the function camel_to_snake uses the reduce() function to iterate over each character in the input string and concatenate it with the previous character, adding an underscore before each uppercase letter (except the first one) to create the snake case string -

from functools import reduce

def camel_to_snake(camel_case_string):
   return reduce(lambda x, y: x + ('_' if y.isupper() else '') + y.lower(), camel_case_string)

camel_case_string = "camelCaseString"
snake_case_string = camel_to_snake(camel_case_string)

print("Camel case string:", camel_case_string)
print("Snake case string:", snake_case_string)

Following is the output of the above code -

Camel case string: camelCaseString
Snake case string: camel_case_string

Using a basic for loop

We can also write a basic loop that goes through each character and manually appends it to the result, adding underscores when needed.

Example

Here is one more example that converts a camel case string to a snake case string, using the for loop to iterate over each character in the input string, converting camel case to snake case -

def camel_to_snake_case(input_str):
   output_str = ""
   for char in input_str:
      if char.isupper():
         output_str += "_" + char.lower()
      else:
         output_str += char
   return output_str

# Example usage
print(camel_to_snake_case("thisIsCamelCase")) # Output: this_is_camel_case

The result produced is as follows -

this_is_camel_case

Using Regular Expression with Group Matching

This approach uses grouping to detect the boundary between lowercase and uppercase letters and inserts underscores there.

Example

Here is another example that uses a regular expression to match and replace camel case. The function camel_to_snake_case uses the re.sub() function with a grouped regex pattern -

import re
def camel_to_snake_case(input_str):
   output_str = re.sub('([a-z0-9])([A-Z])', r'\1_\2', input_str).lower()
   return output_str
# Example usage
print(camel_to_snake_case("thisIsCamelCase")) # Output: this_is_camel_case

We get the output as shown below -

this_is_camel_case