# How to calculate a logistic sigmoid function in Python?

## How to calculate a logistic sigmoid function in Python?

This should do it:

``````import math

def sigmoid(x):
return 1 / (1 + math.exp(-x))
``````

And now you can test it by calling:

``````>>> sigmoid(0.458)
0.61253961344091512
``````

Update: Note that the above was mainly intended as a straight one-to-one translation of the given expression into Python code. It is not tested or known to be a numerically sound implementation. If you know you need a very robust implementation, Im sure there are others where people have actually given this problem some thought.

It is also available in scipy: http://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.logistic.html

``````In [1]: from scipy.stats import logistic

In [2]: logistic.cdf(0.458)
Out[2]: 0.61253961344091512
``````

which is only a costly wrapper (because it allows you to scale and translate the logistic function) of another scipy function:

``````In [3]: from scipy.special import expit

In [4]: expit(0.458)
Out[4]: 0.61253961344091512
``````

If you are concerned about performances continue reading, otherwise just use `expit`.

## Some benchmarking:

``````In [5]: def sigmoid(x):
....:     return 1 / (1 + math.exp(-x))
....:

In [6]: %timeit -r 1 sigmoid(0.458)
1000000 loops, best of 1: 371 ns per loop

In [7]: %timeit -r 1 logistic.cdf(0.458)
10000 loops, best of 1: 72.2 µs per loop

In [8]: %timeit -r 1 expit(0.458)
100000 loops, best of 1: 2.98 µs per loop
``````

As expected `logistic.cdf` is (much) slower than `expit`. `expit` is still slower than the python `sigmoid` function when called with a single value because it is a universal function written in C ( http://docs.scipy.org/doc/numpy/reference/ufuncs.html ) and thus has a call overhead. This overhead is bigger than the computation speedup of `expit` given by its compiled nature when called with a single value. But it becomes negligible when it comes to big arrays:

``````In [9]: import numpy as np

In [10]: x = np.random.random(1000000)

In [11]: def sigmoid_array(x):
....:    return 1 / (1 + np.exp(-x))
....:
``````

(Youll notice the tiny change from `math.exp` to `np.exp` (the first one does not support arrays, but is much faster if you have only one value to compute))

``````In [12]: %timeit -r 1 -n 100 sigmoid_array(x)
100 loops, best of 1: 34.3 ms per loop

In [13]: %timeit -r 1 -n 100 expit(x)
100 loops, best of 1: 31 ms per loop
``````

But when you really need performance, a common practice is to have a precomputed table of the the sigmoid function that hold in RAM, and trade some precision and memory for some speed (for example: http://radimrehurek.com/2013/09/word2vec-in-python-part-two-optimizing/ )

Also, note that `expit` implementation is numerically stable since version 0.14.0: https://github.com/scipy/scipy/issues/3385

#### How to calculate a logistic sigmoid function in Python?

Heres how you would implement the logistic sigmoid in a numerically stable way (as described here):

``````def sigmoid(x):
Numerically-stable sigmoid function.
if x >= 0:
z = exp(-x)
return 1 / (1 + z)
else:
z = exp(x)
return z / (1 + z)
``````

Or perhaps this is more accurate:

``````import numpy as np

def sigmoid(x):
Internally, it implements the same condition as above, but then uses `log1p`.
``````def nat_to_exp(q):
(However, `logaddexp.reduce` could be more accurate.)