[issue11297] Make ChainMap() public in the collections module.

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

FWIW, the new_child() and parents() part of the API was modeled after contexts in ANLTR where they are needed to overcome the limitations of Django's push/pop style which precludes a context from having multiple, independent children at the same time.  The module docstring in the http://code.activestate.com/recipes/577434/ recipe shows how new_child() can be used to easily model both dynamic scoping and nested scoping.

The other advantage of the new_child/parents API over the push/pop API is that it overcomes the occasional templating need to keep two copies of the context (before a push and after a push).

In some ways, it is more difficult to keep track of a mutating chain that is being continuously pushed and popped.  It is simpler to assign a chain to a variable and always know that it is associated with a given template and not have to worry about whether some utility function pushed a new context and failed to pop it when it was done.  A push/pop style introduces the same problems as matching matching malloc() with free() in C.

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<http://bugs.python.org/issue11297>
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