>>> np.arange(2,5)
array([2, 3, 4])
>>> np.arange(2,5)[::-1]
array([4, 3, 2])
>>> np.arange(2,5)[::1]
array([2, 3, 4])

>>> np.prod((2,3))
6
>>> np.prod((2,3,4))
24
>>> np.prod((2))
2

import collections
s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
# defaultdict
d = collections.defaultdict(list)
for k, v in s:
    d[k].append(v)
# Use dict and setdefault
g = {}
for k, v in s:
    g.setdefault(k, []).append(v)
# Use dict
e = {}
for k, v in s:
    e[k] = v

list(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]
>>> list(g.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]
>>> list(e.items())
[('blue', 4), ('red', 1), ('yellow', 3)]
>>> d
defaultdict(<class 'list'>, {'blue': [2, 4], 'red': [1], 'yellow': [1, 3]})
>>> g
{'blue': [2, 4], 'red': [1], 'yellow': [1, 3]}
>>> e
{'blue': 4, 'red': 1, 'yellow': 3}
>>> d.items()
dict_items([('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])])
>>> d["blue"]
[2, 4]
>>> d.keys()
dict_keys(['blue', 'red', 'yellow'])
>>> d.default_factory
<class 'list'>
>>> d.values()
dict_values([[2, 4], [1], [1, 3]])

>>> s = 'mississippi'
>>> d = defaultdict(int)
>>> for k in s:
...     d[k] += 1
...
>>> list(d.items())
[('i', 4), ('p', 2), ('s', 4), ('m', 1)]

>>> from collections import deque
>>> d = deque('ghi')                 # make a new deque with three items
>>> for elem in d:                   # iterate over the deque's elements
...     print elem.upper()
G
H
I

>>> d.append('j')                    # add a new entry to the right side
>>> d.appendleft('f')                # add a new entry to the left side
>>> d                                # show the representation of the deque
deque(['f', 'g', 'h', 'i', 'j'])

>>> d.pop()                          # return and remove the rightmost item
'j'
>>> d.popleft()                      # return and remove the leftmost item
'f'
>>> list(d)                          # list the contents of the deque
['g', 'h', 'i']
>>> d[0]                             # peek at leftmost item
'g'
>>> d[-1]                            # peek at rightmost item
'i'

>>> list(reversed(d))                # list the contents of a deque in reverse
['i', 'h', 'g']
>>> 'h' in d                         # search the deque
True
>>> d.extend('jkl')                  # add multiple elements at once
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> d.rotate(1)                      # right rotation
>>> d
deque(['l', 'g', 'h', 'i', 'j', 'k'])
>>> d.rotate(-1)                     # left rotation
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])

>>> deque(reversed(d))               # make a new deque in reverse order
deque(['l', 'k', 'j', 'i', 'h', 'g'])
>>> d.clear()                        # empty the deque
>>> d.pop()                          # cannot pop from an empty deque
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in -toplevel-
    d.pop()
IndexError: pop from an empty deque

>>> d.extendleft('abc')              # extendleft() reverses the input order
>>> d
deque(['c', 'b', 'a'])

>>> x = np.array([0.2, 6.4, 3.0, 1.6])
>>> bins = np.array([0.0, 1.0, 2.5, 4.0, 10.0])
>>> inds = np.digitize(x, bins)
>>> inds
array([1, 4, 3, 2])
>>> for n in range(x.size):
...   print bins[inds[n]-1], "<=", x[n], "<", bins[inds[n]]
...
0.0 <= 0.2 < 1.0
4.0 <= 6.4 < 10.0
2.5 <= 3.0 < 4.0
1.0 <= 1.6 < 2.5

>>> x = np.array([1.2, 10.0, 12.4, 15.5, 20.])
>>> bins = np.array([0,5,10,15,20])
>>> np.digitize(x,bins,right=True)
array([1, 2, 3, 4, 4])
>>> np.digitize(x,bins,right=False)
array([1, 3, 3, 4, 5])

>>> np.random.uniform(0,1,2)
array([0.6574113 , 0.73053246])

>>> a = np.array([[1, 2], [3, 4]])
>>> b = np.array([[5, 6]])
>>> np.concatenate((a, b), axis=0)
array([[1, 2],
       [3, 4],
       [5, 6]])
>>> np.concatenate((a, b.T), axis=1)
array([[1, 2, 5],
       [3, 4, 6]])
>>> np.concatenate((a, b), axis=None)
array([1, 2, 3, 4, 5, 6]) ### [stack hstack vstack](https://docs.scipy.org/doc/numpy-1.15.0/reference/generated/numpy.hstack.html)