How to have multiple bogus file writers per instruction with?
I read in this answer Is it possible to have an optional / as operator in python? what you might have a dummy file writer with contextmanager
. I want, however, to open several bogus file writers in context with an instruction.
Let's say I create two dummy files: touch a
and touch b
.
Given the first part of the script:
#!/usr/bin/python
from contextlib import contextmanager
# File names
fa="a"
fb="b"
# Dummy file handler
none_context = contextmanager(lambda: iter([None]))()
This addon works on a single file file (it prints 2
):
printing=False
with (open(fa) if printing else none_context) as writter:
print 1 if printing else 2
This also works because we are actually reading the files (it prints 1
):
printing=True
with (open(fa, "r") if printing else none_context) as writter, \
(open(fb, "r") if printing else none_context) as another_writter:
print 1 if printing else 2
However, this doesn't work if we use two dummy files:
printing=False
with (open(fa, "r") if printing else none_context) as writter, \
(open(fb, "r") if printing else none_context) as another_writter:
print 1 if printing else 2
The error is displayed here:
Traceback (most recent call last):
File "dummy_opener.py", line 23, in <module>
with (open(fa, "r") if printing else none_context) as writter, \
File "/usr/lib64/python2.7/contextlib.py", line 19, in __enter__
raise RuntimeError("generator didn't yield")
RuntimeError: generator didn't yield
Why is this happening? And also: how can I get these multiple commands with open
to work with a bogus file author?
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Your code will fail because you already used an iterator on the first call if you call none_context()
in your source block:
none_context = contextmanager(lambda: iter([None]))
printing=False
with open(fa, "r") if printing else none_context() as writter, \
open(fb, "r") if printing else none_context() as another_writter:
print 1 if printing else 2
You can see using your source code that if you add None
for each open source then the code will work as expected:
none_context = contextmanager(lambda: iter([None,None,None]))()
printing=False
with open(fa, "r") if printing else none_context as writter, \
open(fb, "r") if printing else none_context as another_writter,\
open(fb, "r") if printing else none_context as another_writer3:
print 1 if printing else 2
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