Python error when calling NumPy from class method with map

Question

The following code throws me the error:

Traceback (most recent call last):
  File "", line 25, in <module>
    sol = anna.main()
  File "", line 17, in main
    sol = list(map(self.eat, self.mice))
  File "", line 12, in eat
    calc = np.sqrt((food ** 5))
AttributeError: 'int' object has no attribute 'sqrt'

Code:

import numpy as np
#import time

class anaconda():

    def __init__(self):
        self.mice = range(10000)

    def eat(self, food):
        calc = np.sqrt((food ** 5))
        return calc

    def main(self):

        sol = list(map(self.eat, self.mice))
        return sol


if __name__ == '__main__':
    #start = time.time()
    anna = anaconda()
    sol = anna.main()
    print(len(sol))
    #print(time.time() - start)

I believe I made a serious mistake, because it seems like Python interprets the 'np' from NumPy as an integer, but I have no glimpse why that is.

Answer 1

I'll try to add a precise answer to those that have already been given. numpy.sqrt has some limitations that math.sqrt doesn't have.

import math
import numpy  # version 1.13.3

print(math.sqrt(2 ** 64 - 1))
print(numpy.sqrt(2 ** 64 - 1))

print(math.sqrt(2 ** 64))
print(numpy.sqrt(2 ** 64))

returns (with Python 3.5) :

4294967296.0
4294967296.0
4294967296.0
Traceback (most recent call last):
  File "main.py", line 8, in <module>
    print(numpy.sqrt(2 ** 64))
AttributeError: 'int' object has no attribute 'sqrt'

In fact, 2 ** 64 is equal to 18,446,744,073,709,551,616 and, according to the standard of C data types (version C99), the long long unsigned integer type contains at least the range between 0 and 18,446,744,073,709,551,615 included.

The AttributeError occurs because numpy, seeing a type that it doesn't know how to handle (after conversion to C data type), defaults to calling the sqrt method on the object (but that doesn't exist). If we use floats instead of integers then everything will work using numpy:

import numpy  # version 1.13.3

print(numpy.sqrt(float(2 ** 64)))

returns:

4294967296.0

So instead of replacing numpy.sqrt by math.sqrt, you can alternatively replace calc = np.sqrt(food ** 5) by calc = np.sqrt(float(food ** 5)) in your code.

I hope this error will make more sense to you now.

Answer 2

As others have noticed, this boils down to the fact that np.sqrt(7131 ** 5) works but np.sqrt(7132 ** 5) returns an error:

import numpy as np

print(np.sqrt(7131 ** 5))
print(np.sqrt(7132 ** 5))

# 4294138928.9
Traceback (most recent call last):
  File "main.py", line 4, in <module>
    print(np.sqrt(7132 ** 5))
AttributeError: 'int' object has no attribute 'sqrt'

Since np.sqrt docs don't mention any bounds on the argument, I'd consider this a numpy bug.