All programs in the (N)MH suite of programs use the
MHCONTEXT environment variable to find the so-called
context file where the current folder is remembered.
mhlib should do the same, so that it can be used in
combination with the standard (N)MH programs.

Also, when writing the context file, mhlib should
replace the Current-Folder line but keep the other
lines in tact.

The attached patch fixes both problems. It introduces a
new method for the class MH called getcontextfile which
uses the MHCONTEXT environment variable to find the
context file, and it uses the already existing function
updateline to update the context file.

Some questions concerning this patch:
- should I document the new method or should it be an
internal method only?
- should the fix be ported to older Python versions?
With the patch it does behave differently if you have
an MHCONTEXT environment variable.

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The patch looks OK.
  
Regarding your questions: 1) I think the method should be documented; it might be useful to subclasses of MH.  2) New feature, so 2.6 only.

I have added a line to the docstring and I have added a method description to the library reference.  Other than those changes, the new patch is identical to the old.

I can check this in if you want.
File Added: mhlib.patch

If it's a new feature, it should have a test.

This won't happen directly as mhlib has been removed from py3k, but presumably the patch could be reworked to apply to the mailbox module where the MH class now lives.

The mailbox module provides tools for accessing an MH folder, but it does not do anything itself about folder management.  That would be the province of a program using the mailbox module to implement an MUA.  So, since mhlib is no more, this enhancement request is no longer valid.