How to fix Python int too large to convert to C long error

If you’re working with large integer numbers in NumPy, you might get the following error:

OverflowError: Python int too large to convert to C long

This error occurs when the integer number you specified is greater than the size that C long type can handle.

The maximum size that can be handled in C long is as follows:

• The int type is 32-bit which can hold a maximum value of 2147483647
• The int64 type is 64-bit which can hold a maximum value of 9223372036854775807

This tutorial shows an example that causes this error and how to fix it.

How to reproduce this error

Suppose you’re trying to add a large number to a NumPy array as follows:

import numpy as np

arr = np.array(2147483649, dtype=int)

print(arr)

If you installed the 32-bit Python program in Windows, then the integer number 2147483649 will cause an error because it’s greater than the maximum value the int type can handle:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    arr = np.array(2147483649, dtype=int)
OverflowError: Python int too large to convert to C long

On the other hand, if you have a 64-bit Python program installed, integer numbers greater than 9223372036854775807 will cause this error:

import numpy as np

arr = np.array(9223372036854775808, dtype=int)  # ❌

print(arr)

The Numpy int type is equivalent to the long int type in C programming language, which is used internally by Python 2.

In Python 3, the int type is implemented as arbitrary-precision integers, which basically means the integer value is only limited by the available memory in the system.

But the NumPy int type has yet to use the Python 3 implementation, so it’s still limited to the maximum value a long int type can handle.

How to fix this error

If you’re on Windows, then you can change the dtype to int64 to increase the maximum integer value that NumPy can handle:

import numpy as np

arr = np.array(2147483649, dtype=np.int64)

print(arr)  # 2147483649

If you’re already on a 64-bit system, then the maximum integer value you can store in NumPy is 9223372036854775807. Python will raise the error when you try to store a value greater than that.

You can check the maximum allowed integer size in the sys.maxsize constant:

import sys

print(sys.maxsize)  
# 2147483647 / 9223372036854775807

The NumPy int64 type will always be limited by this maximum size, unlike Python 3 int which can hold super big numbers:

# A list in Python 3

x = [9223372036854775899]

print(x)  # [9223372036854775899]

And that’s how you fix the OverflowError: Python int too large to convert to C long error. Happy coding! 🙌