Python - abs vs fabs

Question

I noticed that in python there are two similar looking methods for finding the absolute value of a number:

First

abs(-5) 

Second

import math math.fabs(-5) 

How do these methods differ?

Answer 1

math.fabs() converts its argument to float if it can (if it can't, it throws an exception). It then takes the absolute value, and returns the result as a float.

In addition to floats, abs() also works with integers and complex numbers. Its return type depends on the type of its argument.

In [7]: type(abs(-2)) Out[7]: int  In [8]: type(abs(-2.0)) Out[8]: float  In [9]: type(abs(3+4j)) Out[9]: float  In [10]: type(math.fabs(-2)) Out[10]: float  In [11]: type(math.fabs(-2.0)) Out[11]: float  In [12]: type(math.fabs(3+4j)) --------------------------------------------------------------------------- TypeError                                 Traceback (most recent call last) /home/npe/<ipython-input-12-8368761369da> in <module>() ----> 1 type(math.fabs(3+4j))  TypeError: can't convert complex to float 

Answer 2

Edit: as @aix suggested, a better (more fair) way to compare the speed difference:

In [1]: %timeit abs(5) 10000000 loops, best of 3: 86.5 ns per loop  In [2]: from math import fabs  In [3]: %timeit fabs(5) 10000000 loops, best of 3: 115 ns per loop  In [4]: %timeit abs(-5) 10000000 loops, best of 3: 88.3 ns per loop  In [5]: %timeit fabs(-5) 10000000 loops, best of 3: 114 ns per loop  In [6]: %timeit abs(5.0) 10000000 loops, best of 3: 92.5 ns per loop  In [7]: %timeit fabs(5.0) 10000000 loops, best of 3: 93.2 ns per loop  In [8]: %timeit abs(-5.0) 10000000 loops, best of 3: 91.8 ns per loop  In [9]: %timeit fabs(-5.0) 10000000 loops, best of 3: 91 ns per loop 

So it seems abs() only has slight speed advantage over fabs() for integers. For floats, abs() and fabs() demonstrate similar speed.

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In addition to what @aix has said, one more thing to consider is the speed difference:

In [1]: %timeit abs(-5) 10000000 loops, best of 3: 102 ns per loop  In [2]: import math  In [3]: %timeit math.fabs(-5) 10000000 loops, best of 3: 194 ns per loop 

So abs() is faster than math.fabs().