8/6/2019 Python Alias

1/52

Python

Copyright Software Carpentry 2010

This work is licensed under the Creative Commons Attribution LicenseSee http://software-carpentry.org/license.html for more information.

8/6/2019 Python Alias

2/52

An aliasis a second name for a piece of data

Python Aliasing

8/6/2019 Python Alias

3/52

An aliasis a second name for a piece of dataOften easier (and more useful) than making a

Python Aliasing

8/6/2019 Python Alias

4/52

An aliasis a second name for a piece of dataOften easier (and more useful) than making a

If the data is immutable, aliases don't matter

Python Aliasing

8/6/2019 Python Alias

5/52

An aliasis a second name for a piece of dataOften easier (and more useful) than making a

If the data is immutable, aliases don't matter

Because the data can't change

Python Aliasing

8/6/2019 Python Alias

6/52

8/6/2019 Python Alias

7/52

Aliasing happens whenever one variable's valueis assigned to another variable

Python Aliasing

8/6/2019 Python Alias

8/52

' '

Aliasing happens whenever one variable's valueis assigned to another variable

variablevariablevariablevariable valuevaluevaluevalue

Python Aliasing

first 'isaac'

8/6/2019 Python Alias

9/52

' '

Aliasing happens whenever one variable's valueis assigned to another variable

second = first

variablevariablevariablevariable valuevaluevaluevalue

Python Aliasing

first

second

'isaac'

8/6/2019 Python Alias

10/52

' '

Aliasing happens whenever one variable's valueis assigned to another variable

second = first

variablevariablevariablevariable valuevaluevaluevalue

But as we've already seen

Python Aliasing

first

second

'isaac'

8/6/2019 Python Alias

11/52

' '

Aliasing happens whenever one variable's valueis assigned to another variable

second = first

variablevariablevariablevariable valuevaluevaluevaluefirst = first + ' newton'

But as we've already seen

Python Aliasing

first

second

'isaac'

'isaac newton'

8/6/2019 Python Alias

12/52

But lists are mutable

Python Aliasing

8/6/2019 Python Alias

13/52

But lists are mutable

first = ['isaac']

variablevariablevariablevariable valuevaluevaluevalue

first

Python Aliasing

'isaac'

8/6/2019 Python Alias

14/52

But lists are mutable

first = ['isaac']

=

variablevariablevariablevariable valuevaluevaluevalue

first

Python Aliasing

second

'isaac'

8/6/2019 Python Alias

15/52

But lists are mutable

first = ['isaac']

=

variablevariablevariablevariable valuevaluevaluevalue

first

first = first.append('newton')

printprintprintprint first

['isaac', 'newton']

Python Aliasing

second

'isaac' 'newton'

8/6/2019 Python Alias

16/52

But lists are mutable

first = ['isaac']

=

first = first.append('newton')

printprintprintprint first

['isaac', 'newton']

printprintprintprint second['isaac', 'newton'] variablevariablevariablevariable valuevaluevaluevalue

first

Python Aliasing

second

'isaac' 'newton'

8/6/2019 Python Alias

17/52

But lists are mutable

first = ['isaac']

=

first = first.append('newton')

printprintprintprint first

['isaac', 'newton']

printprintprintprint second['isaac', 'newton'] variablevariablevariablevariable valuevaluevaluevalue

first

Python Aliasing

second

'isaac' 'newton'

n t exp c t ymodify second

8/6/2019 Python Alias

18/52

But lists are mutable

first = ['isaac']

=

first = first.append('newton')

printprintprintprint first

['isaac', 'newton']

printprintprintprint second['isaac', 'newton'] variablevariablevariablevariable valuevaluevaluevalue

first

Python Aliasing

second

'isaac' 'newton'

n t exp c t ymodify second

A side effect

8/6/2019 Python Alias

19/52

Example: use lists of liststo implement a 2D grid

Python Aliasing

8/6/2019 Python Alias

20/52

3

Example: use lists of liststo implement a 2D grid

7

5

5

2

6

7 5 8

5 6 3

3 2 4

Python Aliasing

8

4

3

8/6/2019 Python Alias

21/52

3

Example: use lists of liststo implement a 2D grid

7

5

5

2

6

7 5 8

5 6 3

3 2 4

grid

Python Aliasing

8

4

3

8/6/2019 Python Alias

22/52

3

Example: use lists of liststo implement a 2D grid

7

5

5

2

6

7 5 8

5 6 3

3 2 4

grid[0]

Python Aliasing

8

4

3

8/6/2019 Python Alias

23/52

3

Example: use lists of liststo implement a 2D grid

7

5

5

2

6

7 5 8

5 6 3

3 2 4

grid[0][1]

Python Aliasing

8

4

3

8/6/2019 Python Alias

24/52

# Correct code

grid = []

forforforfor x inininin range(N):

temp = []

forforforfor y inininin range(N):

temp.append(1)

grid.append(temp)

Python Aliasing

8/6/2019 Python Alias

25/52

# Correct code

grid = []

forforforfor x inininin range(N):

temp = []

Outer "spine" of structure

forforforfor y inininin range(N):

temp.append(1)

grid.append(temp)

Python Aliasing

8/6/2019 Python Alias

26/52

# Correct code

grid = []

forforforfor x inininin range(N):

temp = []

forforforfor y inininin range(N):

temp.append(1)

grid.append(temp)

Add N sub-lists to outer list

Python Aliasing

8/6/2019 Python Alias

27/52

# Correct code

grid = []

forforforfor x inininin range(N):

temp = []

forforforfor y inininin range(N):

temp.append(1)

grid.append(temp)

Create a sublist of N 1's

Python Aliasing

8/6/2019 Python Alias

28/52

# Equivalent code

grid = []

forforforfor x inininin range(N):

grid.append([])

forforforfor y inininin range(N):

grid[-1].append(1)

Python Aliasing

8/6/2019 Python Alias

29/52

# Equivalent code

grid = []

forforforfor x inininin range(N):

grid.append([])

forforforfor y inininin range(N):

grid[-1].append(1)

Last element of outer list is the sublist currently

Python Aliasing

e ng e n

8/6/2019 Python Alias

30/52

# Incorrect code

grid = []

EMPTY = []

forforforfor x inininin range(N):

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

Python Aliasing

8/6/2019 Python Alias

31/52

# Incorrect code

grid = []

EMPTY = []

forforforfor x inininin range(N):

# Equivalent code

grid = []

forforforfor x inininin range(N):

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

grid.append([])

forforforfor y inininin range(N):

grid[-1].append(1)

Python Aliasing

8/6/2019 Python Alias

32/52

# Incorrect code

grid = []

EMPTY = []

forforforfor x inininin range(N):

Aren't meaningful variable

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

a good thing?

Python Aliasing

8/6/2019 Python Alias

33/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 0

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

grid

EMPTY

Python Aliasing

8/6/2019 Python Alias

34/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 0

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

grid

EMPTY

Python Aliasing

8/6/2019 Python Alias

35/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 0

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

EMPTY

0

Python Aliasing

8/6/2019 Python Alias

36/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 0

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

EMPTY

0

Python Aliasing

1

8/6/2019 Python Alias

37/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 0

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

EMPTY

2

Python Aliasing

1 1 1

8/6/2019 Python Alias

38/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 1

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

EMPTY

2

Python Aliasing

1 1 1

8/6/2019 Python Alias

39/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

EMPTY = []

x 1

grid.append(EMPTY)

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

EMPTY

2

Python Aliasing

1 1 1

You see the problem...

8/6/2019 Python Alias

40/52

No Aliasing

second = []

Python Aliasing

8/6/2019 Python Alias

41/52

No Aliasing Aliasing

second = [] second = first

Python Aliasing

8/6/2019 Python Alias

42/52

8/6/2019 Python Alias

43/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

forforforfor x inininin range(N):

x 0

.

forforforfor y inininin range(N):

grid[-1].append(1)grid

Python Aliasing

8/6/2019 Python Alias

44/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

forforforfor x inininin range(N):

x 0

.

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

2

Python Aliasing

1 1 1

8/6/2019 Python Alias

45/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

forforforfor x inininin range(N):

x 1

.

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

2

Python Aliasing

1 1 1

8/6/2019 Python Alias

46/52

variablevariablevariablevariable valuevaluevaluevalue

grid = []

forforforfor x inininin range(N):

x 1

.

forforforfor y inininin range(N):

grid[-1].append(1)

y

grid

0

Python Aliasing

1 1 1

1

8/6/2019 Python Alias

47/52

If aliasing can cause bugs, why allow it?

Python Aliasing

8/6/2019 Python Alias

48/52

If aliasing can cause bugs, why allow it?

1. Some languages don't

Python Aliasing

8/6/2019 Python Alias

49/52

If aliasing can cause bugs, why allow it?

1. Some languages don't

Python Aliasing

8/6/2019 Python Alias

50/52

If aliasing can cause bugs, why allow it?

1. Some languages don't

2. Aliasing a million-element list is more efficient

than copying it

Python Aliasing

8/6/2019 Python Alias

51/52

If aliasing can cause bugs, why allow it?

1. Some languages don't

2. Aliasing a million-element list is more efficient

than copying it

3. Sometimes really do want to update a structure

Python Aliasing

8/6/2019 Python Alias

52/52

created by

Greg Wilson

c o er

Copyright Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.