Context inside Do[...]-loop

Question

Consider the following code:

Begin["mycontext`"];
myvar = 7;
Print[Context[]];
Print[Context[myvar]];
End[];

with the output

mycontext`

mycontext`

Now consider the same code inside a Do-loop:

Do[
Begin["mycontext`"];
myvar = 7;
Print[Context[]];
Print[Context[myvar]];
End[];
, {i,1,2}];

which produces the output

mycontext`

Global`

mycontext`

Global`

What is going on here? Why is myvar considered to be in "Global`"? Even if I force the definition to

mycontext`myvar = 7;

it will still be considered to be global. How do I use contexts inside loops correctly?

The whole point of doing this is the following: I have several similar configuration files (that only contain variables) in "./folder1/config.m" and "./folder2/config.m" etc. which I have to apply some manipulations to. My idea is to loop over all folders, use a local context, Get["./folderX/config.m"], write some new values for the variables, and then Save["./folderX/config.m", "mycontext`"].

Example config.m

currentfolderstuff = 0;
anotherfolderstuff = 0;

What I want to do

Do[
(* some sophisticated stuff with many variable names that will pollute my local context *)
a = i^2 + 7^2 + ...;
b = somefunc[i];
Begin["mycontex`"];
Get["./folder"<>ToString[i]<>"/config.m"];
currentfolderstuff = a;
anotherfolderstuff = b;
Save["./folder"<>ToString[i]<>"/newconfig.m", "mycontext`"];
End[];
, {i,1,2}];

Answer 1

Begin["mycontext`"] affects the parser, not the evaluator.

These are separate expressions:

Begin["mycontext`"];

myvar = 7;

Print[Context[]];

Print[Context[myvar]];

End[];

They get parsed and evaluated one by one. Once Begin["mycontext`"]; is evaluated, the new context applies to all subsequent lines.

This is a single expression:

Do[
Begin["mycontext`"];
myvar = 7;
Print[Context[]];
Print[Context[myvar]];
End[];
, {i,1,2}];

It gets parsed as one unit, an evaluation begins only afterwards. Begin["mycontext`"]; does not get a chance to get evaluated before the parsing of myvar.