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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?

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  • $\begingroup$ Just prefix with a backtick. $\endgroup$ – Alan Feb 14 '18 at 15:28
  • $\begingroup$ Btw, there is no magic happening here if you understand how symbols are resolved. Read carefully the documentation of $ContextPath, $Context, Begin and BeginPackage. Your problem is that although Begin changes the $Context, it does not change the current $ContextPath which is why the global x is found first if it is already defined. $\endgroup$ – halirutan Feb 14 '18 at 18:49
  • $\begingroup$ Even if the mycontext` was included early on in $ContextPath, wouldn't the Global`x still be used, as mycontext`x was not defined when the expression "x=4" was interpreted? $\endgroup$ – Daniel Martin Feb 14 '18 at 21:01
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Why not use BeginPackage/Begin? For example:

x=1;

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

{x,foo`Private`}

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  • $\begingroup$ Thanks, this appears to do the trick. $\endgroup$ – Daniel Martin Feb 14 '18 at 17:51
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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 *)
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  • $\begingroup$ Thanks for letting me know about that notation. I'd like to not modify the internal code itself in this case, however. $\endgroup$ – Daniel Martin Feb 14 '18 at 17:33

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