Convert a String to a Number (int, float) in Python
In Python, you can convert a string (str) to a number using int() for an integer or float() for a floating-point number.
Use str() to convert a number to a string.
To format numbers or strings in various styles, such as zero-padded, binary, octal, hexadecimal, or scientific notation, use the format() function or the format() method on string objects.
You can also convert a list of strings to a list of numbers.
Convert a string to int: int()
int() converts a string to an integer (int).
print(int('100'))
print(type(int('100')))
# 100
# <class 'int'>
If the string contains a decimal point (.) or a comma (,), int() will raise an error.
# print(int('1.23'))
# ValueError: invalid literal for int() with base 10: '1.23'
# print(int('10,000'))
# ValueError: invalid literal for int() with base 10: '10,000'
To handle comma-separated strings, replace commas with an empty string ('') using replace().
print(int('10,000'.replace(',', '')))
# 10000
For more on replace(), see the following article:
Starting with versions released after September 7, 2022 (Python 3.11, 3.10.7, 3.9.14, 3.8.14, 3.7.14, and later), integer string conversion is limited to 4300 digits by default. This will be explained in more detail later.
Convert a string to float: float()
float() converts a string to a floating-point number (float).
print(float('1.23'))
print(type(float('1.23')))
# 1.23
# <class 'float'>
Strings that contain only a fractional part or represent a whole number can also be converted using float().
print(float('.23'))
# 0.23
print(float('100'))
print(type(float('100')))
# 100.0
# <class 'float'>
Convert a binary, octal, or hexadecimal string to int
By specifying the base as the second argument to int(), you can convert strings in binary, octal, or hexadecimal notation to integers.
print(int('100', 2))
print(int('100', 8))
print(int('100', 16))
# 4
# 64
# 256
By default, the base is 10, treating the string as a decimal number.
print(int('100', 10))
print(int('100'))
# 100
# 100
If you set the base to 0, the function automatically detects the format based on the string’s prefix (0b, 0o, 0x, 0B, 0O, 0X).
print(int('0b100', 0))
print(int('0o100', 0))
print(int('0x100', 0))
# 4
# 64
# 256
Prefixes and hexadecimal digits can be either uppercase or lowercase.
print(int('FF', 16))
print(int('ff', 16))
# 255
# 255
print(int('0xFF', 0))
print(int('0XFF', 0))
print(int('0xff', 0))
print(int('0Xff', 0))
# 255
# 255
# 255
# 255
For more about binary, octal, and hexadecimal conversions, see the following article:
Convert a scientific notation string to float
Strings in scientific notation can be converted to floating-point numbers using float().
print(float('1.23e-4'))
print(type(float('1.23e-4')))
# 0.000123
# <class 'float'>
print(float('1.23e4'))
print(type(float('1.23e4')))
# 12300.0
# <class 'float'>
Both lowercase e and uppercase E are accepted as the exponent indicator.
print(float('1.23E-4'))
# 0.000123
Integer string conversion length limitation
Starting with versions released after September 7, 2022 (Python 3.11, 3.10.7, 3.9.14, 3.8.14, 3.7.14, and later), integer string conversion is limited to 4300 digits by default.
This limitation aims to prevent DoS attacks that exploit the O(n^2) complexity of integer string conversions.
If you attempt to convert a string that exceeds this limit using int() or str(), a ValueError is raised.
i = int('1' * 5)
print(i)
# 11111
i = int('1' * 4300)
# i = int('1' * 4301)
# ValueError: Exceeds the limit (4300 digits) for integer string conversion: value has 4301 digits; use sys.set_int_max_str_digits() to increase the limit
s = str(10**5)
print(s)
# 100000
s = str(10**4299)
# s = str(10**4300)
# ValueError: Exceeds the limit (4300 digits) for integer string conversion; use sys.set_int_max_str_digits() to increase the limit
This also applies to internal string conversions in functions like print() and repr().
i = 10**10000
# print(i)
# ValueError: Exceeds the limit (4300 digits) for integer string conversion; use sys.set_int_max_str_digits() to increase the limit
As explained in the next section, conversions using bases that are powers of 2 (binary, octal, hexadecimal, etc.) are not subject to this limitation. For a full list of affected APIs, refer to the official documentation:
Unlimited conversion for bases that are powers of 2
Conversions where the base is a power of 2, such as binary, octal, or hexadecimal, are not limited.
For example, conversions using int(string, base) with bases such as 2, 4, 8, 16, or 32, as well as conversions from integers to strings using bin(), oct(), or hex(), will succeed even if the result exceeds 4300 digits.
i = int('1' * 10000, base=16)
s = hex(10**10000)
Set the limit
You can set the digit limit using sys.set_int_max_str_digits(). Setting it to 0 disables the limit.
import sys
sys.set_int_max_str_digits(1000)
# i = int('1' * 1001)
# ValueError: Exceeds the limit (1000 digits) for integer string conversion: value has 1001 digits; use sys.set_int_max_str_digits() to increase the limit
sys.set_int_max_str_digits(0)
i = int('1' * 100000)
You can also set the limit via the environment variable PYTHONINTMAXSTRDIGITS or the command-line option -X int_max_str_digits. For details, see the official documentation:
As noted earlier, this limitation is a security measure and should be modified with caution, especially in applications that handle external input.