I was experimenting with numpy arrays, I had this very simple example:
list = [2, 3]
list.append(np.array([3.2, 9.0, 7.6]))
And if I do print(list), it prints: [2, 3, array([3.2, 9. , 7.6])]
But if I do print(list[-1]), it prints only [3.2 9. 7.6]
Can anyone expalin this for me, that if I print the numpy array as the last element of my python list, why does it print without the "array", the brackets, and the comas between the numbers?
Many thanks
Objects in python generally have two different methods that return a string representation of themselves for print.
They are __str__() and __repr__(), and they have different uses that you can read about here.
To print something you first have to convert it to a string. Normally print will attempt to use __str__ to make this conversion.
Doing print on a list results in a call on list.__str__(), which then tries to convert all it's internal elements into a string for printing. The call the list uses to convert these elements is __repr__().
So when you print a list, the list gets the __repr__ of each element, but when you print just that element, the print gets the __str__ of that element.
What you're seeing is the difference between __repr__ and __str__ in a numpy array.
x = np.array([1, 2, 3])
x.__str__()
x.__repr__()
Output:
'[1 2 3]'
'array([1, 2, 3])'
This is because a numpy array when it's printed looks like a list.
But try this:
>>> import numpy as np
>>> l = [2, 3]
>>> l.append(np.array([3.2, 9.0, 7.6]))
>>> isinstance(l[-1], list)
False
>>> l2 = [3, 2]
>>> isinstance(l2, list)
True
And you will test that the array is not actually a list.
It seems that it is because print function implicitly calls __str__ on its args.
But if you embedd an object in a list, to represent what is in the list, __repr__ will be call for all elements inside the list:
a = np.linspace(0, 10, 11)
print(a)
# [ 0. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.]
print(repr(a))
# array([ 0., 1., 2., 3., 4., 5., 6., 7., 8., 9., 10.])
what you can do is cast the array to a list when printing and the array won't show up:
print([a]) # not good
# array([ 0., 1., 2., 3., 4., 5., 6., 7., 8., 9., 10.])
print([list(a)]) # better
# [[0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0]]
