Shuffling a list of objects
I have a list of objects in Python and I want to shuffle them. I thought I could use the random.shuffle method, but this seems to fail when the list is of objects. Is there a method for shuffling object or another way around this?
import random
class a:
foo = "bar"
a1 = a()
a2 = a()
b = [a1,a2]
print random.shuffle(b)
This will fail.
random.shuffle should work. Here's an example, where the objects are lists:
from random import shuffle
x = [[i] for i in range(10)]
shuffle(x)
# print x gives [[9], [2], [7], [0], [4], [5], [3], [1], [8], [6]]
# of course your results will vary
Note that shuffle works in place , and returns None.
As you learned the in-place shuffling was the problem. I also have problem frequently, and often seem to forget how to copy a list, too. Using sample(a, len(a)) is the solution, using len(a) as the sample size. See https://docs.python.org/3.6/library/random.html#random.sample for the Python documentation.
Here's a simple version using random.sample() that returns the shuffled result as a new list.
import random
a = range(5)
b = random.sample(a, len(a))
print a, b, "two list same:", a == b
# print: [0, 1, 2, 3, 4] [2, 1, 3, 4, 0] two list same: False
# The function sample allows no duplicates.
# Result can be smaller but not larger than the input.
a = range(555)
b = random.sample(a, len(a))
print "no duplicates:", a == list(set(b))
try:
random.sample(a, len(a) + 1)
except ValueError as e:
print "Nope!", e
# print: no duplicates: True
# print: Nope! sample larger than population
#!/usr/bin/python3
import random
s=list(range(5))
random.shuffle(s) # << shuffle before print or assignment
print(s)
# print: [2, 4, 1, 3, 0]
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