layout: post title: Python’s for loop syntax is more flexible than I thought! date: 2022-12-31 summary: TL;DR - All assignment LHS forms are valid syntax for the for-loop target

I came across an expression like:

for self.batch in batches:
    ...

I had no idea one could use expressions like self.batch as the loop iterator. This works as expected, and right after the last iteration of the loop (assuming there was one), self.batch is set to the last item of batches. It also works if self had no attribute called batch initially.

So I was curious and looked up the relevant portion of the grammar, which is:

# For statement
# -------------

for_stmt:
    | 'for' star_targets 'in' ~ star_expressions ':' [TYPE_COMMENT] block [else_block] 
    | ASYNC 'for' star_targets 'in' ~ star_expressions ':' [TYPE_COMMENT] block [else_block] 

So the left side of the for .. in .. construct can contain “star_targets”:

star_targets:
    | star_target !',' 
    | star_target (',' star_target )* [','] 

star_targets_list_seq: ','.star_target+ [','] 

star_targets_tuple_seq:
    | star_target (',' star_target )+ [','] 
    | star_target ',' 

star_target:
    | '*' (!'*' star_target) 
    | target_with_star_atom

target_with_star_atom:
    | t_primary '.' NAME !t_lookahead 
    | t_primary '[' slices ']' !t_lookahead 
    | star_atom

This is a bit more flexible than being able to use an attribute for the iterator.

1. Using an attribute:

class Foo:
  x = 0

foo = Foo()

for foo.x in range(3):
  print(foo.x)

0
1
2

2. Using a *sequence.

for x, *y, z in ((1, 2, 3), ('bar', 'baz', 'spam')):
    print(x, y, z)

1 [2] 3
bar ['baz'] spam

In this form, very sensibly, you can have at most one *starred, and specifying more than one is a syntax error.

3. Using a slice

x = [1, 2, 3]
for x[:2] in [(0, 0), ('bar', 'baz')]:
  print(x)

[0, 0, 3]
['bar', 'baz', 3]

I think I’ve needed to do what each of the above are doing, but I have always gone about it in a more direct style. I am not sure if these lesser known ways of iteration are immediately more or less readable, but they don’t look too clunky.