What does X.T in Python do?

Question

I am very new to Python. I am using the transpose operator in the numpy package:

>>> import numpy as np
>>> X = np.array([[1,2,3],[4,5,6]])
>>> np.T(X)
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    np.T(X)
AttributeError: 'module' object has no attribute 'T'

Why is it that this is an error, yet X.T works? Furthermore, X.np.T fails. On the other hand, np.fft.fft(X) succeeds, but X.fft.fft fails.

Thanks all!

Solution 1

The numpy.array function returns an ndarray object, so when you call

X = np.array([[1,2,3],[4,5,6]])

the variable X is assigned an ndarray. That object has a T method, which transposes the array.

Calling T like the following:

np.T(X)

doesn't work because the numpy library doesn't have a free-floating function named T that takes an array as an argument, just the method in the ndarray class.

Solution 2

X.T means transpose of X. For example lets say this is our X:

X.T is going to be: