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: ```python class Foo: x = 0

foo = Foo()

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

0 1 2


2. Using a `*sequence`. 

```python
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.

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

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

for *foo.ys[:2], in [(0, 0), (‘bar’, ‘baz’)]: print(foo.ys)