python list pattern matching?

[guthrie]

I want to do a functional like pattern match to get teh first two
elements, and then the rest of an array return value.

For example, assume that perms(x) returns a list of values, and I want
to do this:
seq=perms(x)

a = seq[0]
b = seq[1]
rest = seq[2:]
Of course I can shorten to:
[a,b] = seq[0:2]
rest = seq[2:]

Can I find use some notation to do this?
[a,b,more] = perms(x)
or conceptually:
[a,b,more..] = perms(x)

PROLOG & functional languages do list decomposition so nicely like
this!

 

[Terry Reedy]

I want to do a functional like pattern match to get teh first two
elements, and then the rest of an array return value.

For example, assume that perms(x) returns a list of values, and I want
to do this:
seq=perms(x)

a = seq[0]
b = seq[1]
rest = seq[2:]
Of course I can shorten to:
[a,b] = seq[0:2]
rest = seq[2:]

Can I find use some notation to do this?
[a,b,more] = perms(x)
or conceptually:
[a,b,more..] = perms(x)

>>> a,b,*rest = list(range(10))
>>> a,b,rest (0, 1, [2, 3, 4, 5, 6, 7, 8, 9])
>>> a,*rest,b = 'abcdefgh'
>>> a,rest,b

('a', ['b', 'c', 'd', 'e', 'f', 'g'], 'h')

 

[Chris Rebert]

I want to do a functional like pattern match to get teh first two
elements, and then the rest of an array return value.

For example, assume that perms(x) returns a list of values, and I want
to do this:
   seq=perms(x)

   a = seq[0]
   b = seq[1]
   rest = seq[2:]
Of course I can shorten to:
   [a,b] = seq[0:2]
   rest  = seq[2:]

Can I find use some notation to do this?
   [a,b,more] = perms(x)
or conceptually:
   [a,b,more..] = perms(x)

a,b,*rest = list(range(10))
a,b,rest (0, 1, [2, 3, 4, 5, 6, 7, 8, 9])
a,*rest,b = 'abcdefgh'
a,rest,b

('a', ['b', 'c', 'd', 'e', 'f', 'g'], 'h')

Note that this snazzy new syntax requires Python 3.0+

Cheers,
Chris

 

[Mensanator]

I want to do a functional like pattern match to get teh first two
elements, and then the rest of an array return value.

For example, assume that perms(x) returns a list of values, and I want
to do this:
    seq=perms(x)

    a = seq[0]
    b = seq[1]
    rest = seq[2:]
Of course I can shorten to:
    [a,b] = seq[0:2]
    rest  = seq[2:]

Can I find use some notation to do this?
    [a,b,more] = perms(x)
or conceptually:
    [a,b,more..] = perms(x)

perms = 'abcdefghijklmnopqrstuvwxyz'
[a,b],more = perms[0:2],perms[2:]
a 'a'
b 'b'
more

'cdefghijklmnopqrstuvwxyz'



PROLOG & functional languages do list decomposition so nicely like
this!

 

[bearophileHUGS]

Terry Reedy:

 >>> a,b,*rest = list(range(10))
 >>> a,b,rest
(0, 1, [2, 3, 4, 5, 6, 7, 8, 9])
 >>> a,*rest,b = 'abcdefgh'
 >>> a,rest,b
('a', ['b', 'c', 'd', 'e', 'f', 'g'], 'h')

For the next few years I generally suggest to specify the Python
version too (if it's 2.x or 3.x).
This is Python 3.x.

Bye,
bearophile

 

[Steven D'Aprano]

That fails in Python 2.5 and 2.6.
File "<stdin>", line 1
a,b,*rest = list(range(10))
^
SyntaxError: invalid syntax

 

[guthrie]

Many thanks to all; perfect solution!

 

[Peter Otten]

The list() call is superfluous.

 

[Terry Reedy]

The list() call is superfluous.

Agreed, even in Py3 when range() returns a range object rather than a list.