Python Int to String with Leading Zeros

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To convert an integer i to a string with leading zeros so that it consists of 5 characters, use the format string f'{i:05d}'. The d flag in this expression defines that the result is a decimal value. The str(i).zfill(5) accomplishes the same string conversion of an integer with leading zeros.

Challenge: Given an integer number. How to convert it to a string by adding leading zeros so that the string has a fixed number of positions.

Example: For integer 42, you want to fill it up with leading zeros to the following string with 5 characters: '00042'.

In all methods, we assume that the integer has less than 5 characters.

Method 1: Format String

The first method uses the format string feature in Python 3+ called f-strings or replacement fields.

๐Ÿ’ก Info: In Python, f-strings allow for the embedding of expressions within strings by prefixing a string with the letter "f" or "F" and enclosing expressions within curly braces {}. The expressions within the curly braces in the f-string are evaluated, and their values are inserted into the resulting string. This allows for a concise and readable way to include variable values or complex expressions within string literals.

The following f-string converts an integer i to a string while adding leading zeros to a given integer:

# Integer value to be converted
i = 42

# Method 1: Format String
s1 = f'{i:05d}'
# 00042

The code f'{i:05d}' places the integer i into the newly created string. However, it tells the format language to fill the string to 5 characters with leading '0's using the decimal system.

This is the most Pythonic way to accomplish this challenge.

Method 2: zfill()

Another readable and Pythonic way to fill the string with leading 0s is the string.zfill() method.

# Method 2: zfill()
s2 = str(i).zfill(5)
# 00042

The method takes one argument and that is the number of positions of the resulting string. Per default, it fills with 0s.

Python How to Pad Zeros to a String?

You can check out the following video tutorial from Finxter Adam:

Python How to Pad Zeros to a String Video

Method 3: List Comprehension

Many Python coders don’t quite get the f-strings and the zfill() method shown in Methods 2 and 3. If you don’t have time learning them, you can also use a more standard way based on string concatenation and list comprehension.

# Method 3: List Comprehension
s3 = str(i)
n = len(s3)
s3 = '0' * (5-len(s3)) + s3

You first convert the integer to a basic string. Then, you create the prefix of 0s, you need to fill it up to n=5 characters and concatenate it to the integer’s string representation. The asterisk operator creates a string of 5-len(s3) zeros here.

If you want to learn how to add trailing zeros instead of leading zeros, check out this article on the Finxter blog.

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