Why is a symbol still found after changing its context?

Question

It seems that we can change the context of symbols:

In[1]:= x = 1
Out[1]= 1

In[2]:= Context[x] = "foo`";

But then x is still interpreted as foo`x (and not Global`x) even though foo` is not in $ContextPath.

In[3]:= x
Out[3]= 1

In[4]:= foo`x
Out[4]= 1

The behaviour is not consistent though. At other times I randomly get this when evaluating the same sequence of expressions in a new kernel:

In[1]:= x = 1
Out[1]= 1

In[2]:= Context[x] = "foo`";

In[3]:= x
Out[3]= x

In[4]:= foo`x
Out[4]= 1

I see this random, erratic behaviour in M9, 10 and 10.1. In M8, x is not seen any more as soon as its context is changed, i.e. it behaves as one would expect.

Questions:

• Why the random behaviour?
• Is it documented or discussed anywhere that Context an be changed?

Answer 1

I think this is a caching problem. Considering the examples in the Update documentation it certainly makes sense that this could be the case. Although Update appears to have no effect I have found something that does: Information.

x = 1;
Context[x] = "foo`";

x
foo`x
Context[x]

1

1

"foo`"

?x

foo`x

x=1

x
foo`x
Context[x]

x

1

"Global`"

• On my system this is reliable; I get the same output every time I run it.

• Rather surprisingly Information returns the definition for foo`x but after it is used, such as if it is used again, x points to Global`x.

• It does not seem to matter if x is involved; I can for example replace ?x with ?Plus in the lines above and it behaves the same way.

Answer 2

No erratic behavior on my computer M9. However a simple workaround could be considered that achieves its purpose.

Workaround for moving a symbol from one context to another can be achieved by: For example, create symbol in the context Global and move it to foo`

    x = 1;           (*create x in Global` *)
    foo`x = x;       (*create x in foo` and copy Global`x *)
    Remove @ x       (*remove x from Global` it will only exist in foo`*)
    {x, foo`x}
    (* {x,1} *)

Moving it back to Global`

    x=foo`x;
    Remove @ foo`x
    {x,foo`x}
    (* {1,foo`x} *) 

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