Unzipping and the * Operator

Explain why unpack() returns different results in Lua

Change

primes[idx] = n

to

primes[#primes+1] = n

The reason is that idx is not sequential as not every number is a prime.

Unzipping and the * operator

When used like this, the * (asterisk, also know in some circles as the "splat" operator) is a signal to unpack arguments from a list. See http://docs.python.org/tutorial/controlflow.html#unpacking-argument-lists for a more complete definition with examples.

Why does x,y = zip(*zip(a,b)) work in Python?

The asterisk in Python is documented in the Python tutorial, under Unpacking Argument Lists.

Multiple assignments via walrus := operator?

Iterable packing and unpacking is one difference between = and :=, with only the former supporting them. As found in PEP-572:

# Equivalent needs extra parentheses
loc = x, y  # Use (loc := (x, y))
info = name, phone, *rest  # Use (info := (name, phone, *rest))

# No equivalent
px, py, pz = position
name, phone, email, *other_info = contact

Visually Representing X and Y Values

Perhaps you could try something like this (also see):

import numpy as np:
xs=[]; ys=[]
for x,y in xy_list:
  xs.append(x)
  ys.append(y)
xs=np.asarray(xs)
ys=np.asarray(ys)
plot(xs,ys,'ro')

Maybe not the most elegant solution, but it should work. Cheers, Trond

Making a list comprehension for a dict within a list within a list

Animals = [[d['animal'] for d in sub] for sub in MyList]
Colors = [[d['color'] for d in sub] for sub in MyList]

Gives the desired result:

[['A', 'B'], ['C', 'D']]
[['blue', 'red'], ['blue', 'Y']]  # No second 'red'.

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What I have done here is take each sub-list, then each dictionary, and then access the correct key.

Python: TypeError: argument after * must be a sequence

You need to add a comma - , - after your variable s. Sending just s to args=() is trying to unpack a number of arguments instead of sending just that single arguement.

So you'd have threading.Thread(target=send, args=(s,)).start()

Also the splat - * - operator might be useful in this question explaining it's usage and unzipping arguments in general

How to unpack tuples in nested list?

>>> a = [ [(2, 3, 5), (3, 4, 6), (4, 5, 7), (1, 1, 1), (1, 2, 3)],
...       [(4, 9, 2), (8, 8, 0), (3, 5, 1), (2, 6, 8), (2, 4, 8)],
...       [(8, 7, 5), (2, 5, 1), (9, 2, 2), (4, 5, 1), (0, 1, 9)]]
>>> zip(*(zip(*x) for x in a))
[((2, 3, 4, 1, 1), (4, 8, 3, 2, 2), (8, 2, 9, 4, 0)), ((3, 4, 5, 1, 2), (9, 8, 5, 6, 4), (7, 5, 2, 5, 1)), ((5, 6, 7, 1, 3), (2, 0, 1, 8, 8), (5, 1, 2, 1, 9))]

>>> for row in _:
...     print row
... 
((2, 3, 4, 1, 1), (4, 8, 3, 2, 2), (8, 2, 9, 4, 0))
((3, 4, 5, 1, 2), (9, 8, 5, 6, 4), (7, 5, 2, 5, 1))
((5, 6, 7, 1, 3), (2, 0, 1, 8, 8), (5, 1, 2, 1, 9))

If it must be lists

>>> map(list, zip(*(map(list, zip(*x)) for x in a)))
[[[2, 3, 4, 1, 1], [4, 8, 3, 2, 2], [8, 2, 9, 4, 0]], [[3, 4, 5, 1, 2], [9, 8, 5, 6, 4], [7, 5, 2, 5, 1]], [[5, 6, 7, 1, 3], [2, 0, 1, 8, 8], [5, 1, 2, 1, 9]]]
>>> for row in _:
...     print row
... 
[[2, 3, 4, 1, 1], [4, 8, 3, 2, 2], [8, 2, 9, 4, 0]]
[[3, 4, 5, 1, 2], [9, 8, 5, 6, 4], [7, 5, 2, 5, 1]]
[[5, 6, 7, 1, 3], [2, 0, 1, 8, 8], [5, 1, 2, 1, 9]]

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