Abstract

This PEP proposes a change to iterable unpacking syntax, allowing tospecify a “catch-all” name which will be assigned a list of all itemsnot assigned to a “regular” name.

An example says more than a thousand words:

>>>a,*b,c=range(5)>>>a0>>>c4>>>b[1, 2, 3]

Rationale

Many algorithms require splitting a sequence in a “first, rest” pair.With the new syntax,

first,rest=seq[0],seq[1:]

is replaced by the cleaner and probably more efficient:

first,*rest=seq

For more complex unpacking patterns, the new syntax looks evencleaner, and the clumsy index handling is not necessary anymore.

Also, if the right-hand value is not a list, but an iterable, ithas to be converted to a list before being able to do slicing; toavoid creating this temporary list, one has to resort to

it=iter(seq)first=it.next()rest=list(it)

Specification

A tuple (or list) on the left side of a simple assignment (unpackingis not defined for augmented assignment) may contain at most oneexpression prepended with a single asterisk (which is henceforthcalled a “starred” expression, while the other expressions in thelist are called “mandatory”). This designates a subexpression thatwill be assigned a list of all items from the iterable being unpackedthat are not assigned to any of the mandatory expressions, or anempty list if there are no such items.

For example, ifseq is a sliceable sequence, all the followingassignments are equivalent ifseq has at least two elements:

a,b,c=seq[0],list(seq[1:-1]),seq[-1]a,*b,c=seq[a,*b,c]=seq

It is an error (as it is currently) if the iterable doesn’t containenough items to assign to all the mandatory expressions.

It is also an error to use the starred expression as a loneassignment target, as in

*a=range(5)

This, however, is valid syntax:

*a,=range(5)

Note that this proposal also applies to tuples in implicit assignmentcontext, such as in afor statement:

fora,*bin[(1,2,3),(4,5,6,7)]:print(b)

would print out

[2,3][5,6,7]

Starred expressions are only allowed as assignment targets, using themanywhere else (except for star-args in function calls, of course) is anerror.

Implementation

star_expr:['*']expr

comparison:star_expr(comp_opstar_expr)*exprlist:star_expr(','star_expr)*[',']

Acceptance

After a short discussion on the python-3000 list[1], the PEP wasaccepted by Guido in its current form. Possible changes discussedwere:

• Only allow a starred expression as the last item in the exprlist.This would simplify the unpacking code a bit and allow for thestarred expression to be assigned an iterator. This behavior wasrejected because it would be too surprising.
• Try to give the starred target the same type as the sourceiterable, for example,b ina,*b='hello' would beassigned the string'ello'. This may seem nice, but isimpossible to get right consistently with all iterables.
• Make the starred target a tuple instead of a list. This would beconsistent with a function’s*args, but make further processingof the result harder.

References

Copyright

This document has been placed in the public domain.