evaluation - Why the Block command does not forget the $ContextPath variable

I do not understand this behavior:

method[args_] :=
    (* forget the context path *)
    Block[{$ContextPath},
            (* should given nothing, right? *)
            Print["cpath=", $ContextPath];
            ...

but when evaluated it gives the full context path (i.e. it does not forgets it). Why is that? This is against everything I (seem to) know about the Block function.

Answer

If I try this, everything gets removed except for {System`,Global`}:

In[1]:= $ContextPath

Out[1]= {"PacletManager`", "QuantityUnits`", "WebServices`", "System`", "Global`"}

In[2]:= Block[{$ContextPath}, Print[$ContextPath]]

During evaluation of In[2]:= {System`,Global`}

My guess is that this is a special exception which is implemented to make $ContextPath usable like this:

Block[{$ContextPath}, Needs["SomePackage`"]]

This loads the package without adding it to the context path. It's quite useful with Combinatorica in version 8 and later.

It is in fact possible to remove System` and Global` from $ContextPath but this just breaks the system completely and would also make it impossible to load any packages. You can (in a new session, where you won't lose work!!) try $ContextPath = {} and render the system unusable if you are using a notebook interface. The reason why package loading would be broken is that System` context symbols would need to be referenced by the full name, as in System`Sin, which packages don't do.

So I think this is a special exception to the behaviour of Block/$ContextPath that makes the above use case possible.

Comments

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