Alias

Python
Aliasing
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.

An alias is a second name for a piece of data
Python Aliasing

Often easier (and more useful) than making a
sseeccoonndd ccooppyy
Python Aliasing

If the data is immutable, aliases don't matter
Python Aliasing

Because the data can't change
Python Aliasing

Because the data can't change
But if data can change, aliases can result in a lot
of hard-to-find bugs
Python Aliasing

Aliasing happens whenever one variable's value
is assigned to another variable
Python Aliasing

first = 'isaac'
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first 'isaac'
Python Aliasing

first = 'isaac'
second = first
first
second
'isaac'
Python Aliasing

first = 'isaac'
second = first
But as we've already seen…
first
second
'isaac'
Python Aliasing

first = 'isaac'
second = first
But as we've already seen…
first = first + ' newton'
first
second
'isaac'
'isaac newton'
Python Aliasing

But lists are mutable
Python Aliasing

first = ['isaac']
first
'isaac'
Python Aliasing

first = ['isaac']
second = first
first
second
'isaac'
Python Aliasing

first = ['isaac']
second = first
first = first.append('newton')
pppprrrriiiinnnntttt first
['isaac', 'newton']
first
second
'isaac' 'newton'
Python Aliasing

first = ['isaac']
second = first
['isaac', 'newton']
pppprrrriiiinnnntttt second
['isaac', 'newton']
first
second
'isaac' 'newton'
Python Aliasing

first = ['isaac']
second = first
['isaac', 'newton']
['isaac', 'newton']
first
Didn't explicitly
second
'isaac' 'newton'
modify second
Python Aliasing

first = ['isaac']
second = first
['isaac', 'newton']
['isaac', 'newton']
first
Didn't explicitly
second
'isaac' 'newton'
modify second
A side effect
Python Aliasing

Example: use lists of lists
to implement a 2D grid
Python Aliasing

3
5
7
2
6
5
7 5 8
5 6 3
3 2 4
4
3
8
Python Aliasing

3
5
7
2
6
5
7 5 8
5 6 3
3 2 4
grid
4
3
8
Python Aliasing

3
5
7
2
6
5
7 5 8
5 6 3
3 2 4
grid[0]
4
3
8
Python Aliasing

3
5
7
2
6
5
7 5 8
5 6 3
3 2 4
grid[0][1]
4
3
8
Python Aliasing

# Correct code
grid = []
ffffoooorrrr x iiiinnnn range(N):
temp = []
ffffoooorrrr y iiiinnnn range(N):
temp.append(1)
grid.append(temp)
Python Aliasing

# Correct code
grid = []
temp = []
Outer "spine" of structure
temp.append(1)
grid.append(temp)
Python Aliasing

# Correct code
grid = []
temp = []
temp.append(1)
grid.append(temp)
Add N sub-lists to outer list
Python Aliasing

# Correct code
grid = []
temp = []
temp.append(1)
grid.append(temp)
Create a sublist of N 1's
Python Aliasing

# Equivalent code
grid = []
grid.append([])
grid[-1].append(1)
Python Aliasing

# Equivalent code
grid = []
grid.append([])
grid[-1].append(1)
Last element of outer list is the sublist currently
being filled in
Python Aliasing

# Incorrect code
grid = []
EMPTY = []
grid.append(EMPTY)
grid[-1].append(1)
Python Aliasing

# Incorrect code
grid = []
EMPTY = []
# Equivalent code
grid = []
grid.append(EMPTY)
grid[-1].append(1)
grid.append([])
grid[-1].append(1)
Python Aliasing

# Incorrect code
grid = []
EMPTY = []
Aren't meaningful variable
names supposed to be
grid.append(EMPTY)
grid[-1].append(1)
a good thing?
Python Aliasing

grid = []
EMPTY = []
x 0
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
Python Aliasing

grid = []
EMPTY = []
x
y
0
0
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
Python Aliasing

grid = []
EMPTY = []
x
y
0
0
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
1
Python Aliasing

grid = []
EMPTY = []
x
y
0
2
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
1 1 1
Python Aliasing

grid = []
EMPTY = []
x
y
1
2
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
1 1 1
Python Aliasing

grid = []
EMPTY = []
x
y
1
2
grid.append(EMPTY)
grid[-1].append(1)
grid
EMPTY
1 1 1
You see the problem...
Python Aliasing

No Aliasing
ffiirrsstt == [[]]
second = []
Python Aliasing

No Aliasing Aliasing
ffiirrsstt == [[]] ffiirrsstt == [[]]
second = [] second = first
Python Aliasing

grid = []
grid.append([])
x 0
grid[-1].append(1)
grid
Python Aliasing

grid = []
grid.append([])
x
y
0
2
grid[-1].append(1)
grid
1 1 1
Python Aliasing

grid = []
grid.append([])
x
y
1
2
grid[-1].append(1)
grid
1 1 1
Python Aliasing

grid = []
grid.append([])
x
y
1
0
grid[-1].append(1)
grid
1 1 1
1
Python Aliasing

If aliasing can cause bugs, why allow it?
Python Aliasing

1. Some languages don't
Python Aliasing

Or at lleeaasstt aappppeeaarr nnoott ttoo
Python Aliasing

2. Aliasing a million-element list is more efficient
than copying it
Python Aliasing

2. Aliasing a million-element list is more efficient
than copying it
3. Sometimes really do want to update a structure
in place
Python Aliasing

created by
Greg Wilson
October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.

Alias

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