Convert string to int in Python

Key takeaways:

  • Use Python’s built-in int() function to easily convert numeric strings to integers, e.g., int("123").

  • The int() function can convert strings representing numbers in various bases, such as binary (int("1010", 2)) or hexadecimal (int("1F", 16)).

Converting a string to an integer is a common task in Python programming. Whether we’re processing user input, parsing data, or working on computations, knowing how to handle this operation efficiently is crucial.

A string is given to the int() function
A string is given to the int() function
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Using the int() function

The most straightforward way to convert a string to an integer is by using Python’s built-in int() function.

Note: The function returns 0 if no arguments are passed.

Syntax

int(x, base=10)
Syntax parameters for int()
Syntax parameters for int()

The int() function takes a string (or any object) representing a number and converts it into an integer. The function performs the conversion without errors if the string contains valid numeric characters.

Here’s a breakdown of the parameters it takes:

  • x: The value to convert into an integer. It can be:

    • A number (e.g., float, bool)

    • A string representing an integer (e.g., "42")

    • A string representing a number in a different base (e.g., "1010" in binary)

  • base: The base of the number system for conversion.

    • If x is a string, base specifies the number system (e.g., 2 for binary, 16 for hexadecimal, 8 for octal).

    • If x is a number (not a string), base is ignored.

print(int("42"))  # Output: 42

Converting strings with different bases in Python

Python provides built-in functions to convert strings representing numbers in different bases (binary, octal, hexadecimal) into integers and vice versa.

# Converting a binary string to an integer
binary_str = "1010"  # Binary representation of 10
binary_num = int(binary_str, 2)
print("Binary to Integer:", binary_num)
# Converting an octal string to an integer
octal_str = "12"  # Octal representation of 10
octal_num = int(octal_str, 8)
print("Octal to Integer:", octal_num)
# Converting a hexadecimal string to an integer
hex_str = "A"  # Hex representation of 10
hex_num = int(hex_str, 16)
print("Hexadecimal to Integer:", hex_num)

Handling invalid input strings in Python

When converting strings to numbers, invalid inputs (such as non-numeric characters) can cause errors. We can handle such cases using try-except and isdigit() method.

1. Handling invalid input using try-except

The try block attempts to convert the string to an integer. If the string contains invalid characters (like letters or decimals), a ValueError is raised and caught in the except block. An error message is printed, and None is returned.

def safe_convert_to_int(s):
    """Converts a string to an integer safely using try-except."""
    try:
        return int(s)  # Attempt conversion
    except ValueError:
        print(f"Error: '{s}' is not a valid integer.")
        return None
# Test cases
inputs = ["123", "42a", "98.6", "-50", "ten"]
for input_str in inputs:
    result = safe_convert_to_int(input_str)
    print(f"Input: '{input_str}' → Output: {result}")

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2. Handling invalid input using str.isdigit()

isdigit() checks if the string consists only of digits (0-9). It does not support negative numbers ("-50" will fail). If isdigit() returns True, the string is converted to an integer. If False, an error message is printed, and None is returned.

def safe_convert_with_isdigit(s):
    """Converts a numeric string to an integer using isdigit()."""
    if s.isdigit():
        return int(s)
    else:
        print(f"Error: '{s}' is not a purely numeric string.")
        return None
# Test cases
inputs = ["123", "42a", "98.6", "-50", "ten"]
for input_str in inputs:
    result = safe_convert_with_isdigit(input_str)
    print(f"Input: '{input_str}' → Output: {result}")

Conclusion

The int () function in Python makes converting a string to an integer simple and versatile. We can easily handle various scenarios by incorporating techniques such as error handling, stripping whitespace, and validating input.

Frequently asked questions

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What does int() do in Python?

The int() function in Python converts a value to an integer.

How do you convert an int to a string in Python f-string?

An f-string (formatted string literal) is a way to embed expressions inside string literals using curly braces {}. It is prefixed with f or F and allows for easier and more readable string formatting.

You can convert an integer to a string within an f-string by simply placing the integer variable within curly braces {}.

my_int = 42
my_string = f"The answer is: {my_int}" 
print(my_string)  # Output: The answer is: 42

What is toString() in Python?

Python does not have a built-in method called toString(). We can implement toString() to convert a value to a string in Python.

Can we convert string to int in Python?

Yes, you can convert a string to an integer in Python using int():

integer_num = int("100")  # Output: 100

What happens if the string contains non-numeric characters?

The int() function will raise a ValueError if the string contains non-numeric characters (except for leading/trailing spaces, a sign, or a valid base prefix).

Example:

int("123abc")  # Raises ValueError

Can int() handle strings with leading or trailing spaces?

Yes, int() automatically ignores leading and trailing spaces before conversion.

Example:

print(int("  42  "))  # Output: 42

Is there a way to check if a string can be converted to an integer before using int()?

Yes, use str.isdigit() or a try-except block.

Example:

s = "123"
if s.isdigit():
    print(int(s))  # Output: 123

Or using try-except:

try:
    num = int(s)
    print(num)
except ValueError:
    print("Cannot convert to int")

Can I convert a floating-point string to an integer?

Yes, but you must first convert it to a float and then to an integer (which removes the decimal part).

Example:

print(int(float("12.34")))  # Output: 12

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