Assigning variables whilst in a context

Question

When working in a context, I would like all variable assignments to happen on symbols in the new context.

This doesn't normally happen. For example,

Quit[]
x=3
Begin@"mycontext`"

, then

x=4
?? x

produces a value of 4 for Global`x, rather than 3 for Global`x and 4 for mycontext`x as I would prefer in this case. This is because the Global meaning of x is used throughout (because the symbol x has appeared earlier in the Global context).

Is there any code I can add between these two blocks to make this happen? When x is referenced outside of an assignment, I would like x to be interpreted as normal.

I could indeed assign via mycontext`x=4, but what I actually want to do is to insert some context-starting/context-ending commands before/after a block of existing code, such that all variables inside the code will be treated as local, whilst avoiding the downsides of Block or Module. Namely, the downsides are that

1. I would have to record each variable name in the first argument of Block/Module,
2. I would no longer be able to evaluate the code line-by-line in the notebook.

Is this possible, and if so would there be any ugly side-effects?

Answer 1

Why not use BeginPackage/Begin? For example:

x=1;

BeginPackage["foo`"];
Begin["`Private`"];
Print[{x, Context[x]}];
End[];
EndPackage[]

{x,foo`Private`}

Answer 2

You can prefix symbols with a backtick in order to put them in the current context.

x = 1  (* global context *)
`x = 2  (* global context *)
Begin["xxx`"]
x = 5  (* global context (because exists) *)
`x = 6  (* current context *)
End[]
x  (* 5 *)