>>> from itertools import chain
>>>
>>> class TestCM:
...     def __init__(self, it):
...         self.it = it
...     def __enter__(self):
...         print('entering cm for', self.it)
...         return self
...     def __exit__(self, exc_type, exc_val, exc_tb):
...         print('exiting cm for', self.it)
...     def __iter__(self):
...         yield from self.it
...
>>>
>>> def itercm(cm):
...     with cm:
...         yield from cm
...
>>> seq = (itercm(TestCM(s)) for s in ['foo', 'bar', 'baz'])
>>> for c in chain.from_iterable(seq):
...     print(c)
...
entering cm for foo
f
o
o
exiting cm for foo
entering cm for bar
b
a
r
exiting cm for bar
entering cm for baz
b
a
z
exiting cm for baz
>>>
>>>
>>> # exit is called in case of errors
>>> seq = (itercm(TestCM(s)) for s in ['foo', ('a', 3), 'baz'])
>>> for c in chain.from_iterable(seq):
...     print(c.upper())
...
entering cm for foo
F
O
O
exiting cm for foo
entering cm for ('a', 3)
A
exiting cm for ('a', 3)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: 'int' object has no attribute 'upper'
>>>
>>>
>>> import re
>>>
>>> def grep(pattern, fnames):
...     lines = chain.from_iterable(itercm(open(f)) for f in fnames)
...     matches = (line for line in lines if re.search(pattern, line))
...     for line in matches:
...         print(line, end='')
...
>>> grep('^def', ['itercm.py']*3)
def itercm(cm):
def grep(pattern, fnames):
def itercm(cm):
def grep(pattern, fnames):
def itercm(cm):
def grep(pattern, fnames):
>>>