Question

I wish to intercept assigning new values for the In variable.

I have tried to do this by defining UpValues for In but it does not help in this case:

Unprotect[In];
In /: Set[In[line_], expr_] /; ! TrueQ[$InsideSet] := 
 Block[{$InsideSet = True}, 
  Print[HoldForm@HoldForm[expr]; Set[In[line], expr]]]
In /: SetDelayed[In[line_], expr_] /; ! TrueQ[$InsideSet] := 
 Block[{$InsideSet = True}, 
  Print[HoldForm@HoldForm[expr]; SetDelayed[In[line], expr]]]

Is it possible to intercept it?

P.S. This question has arisen as a part of previous question on the stage when Mathematica creates new Symbols.

EDIT

I would wish to intercept explicitly the assignment new DownValue for the In variable. $Pre executes after this assignment and after creating all new Symbols in the current $Context:

In[1]:= $Pre := (Print[Names["`*"]]; 
   Print[DownValues[In][[All, 1]]]; ##) &

In[2]:= a

During evaluation of In[2]:= {a}

During evaluation of In[2]:= {HoldPattern[In[1]],HoldPattern[In[2]]}

Out[2]= a

Answer 1

Have you looked at $Pre and $PreRead?

$Pre is a global variable whose value, if set, is applied to every input expression.

$PreRead is a global variable whose value, if set, is applied to the text or box form of every input expression before it is fed to Mathematica.

UPDATE (now with better example)

In[1]:= $Pre = 
  Function[{x}, Print["In[",$Line,"] is: ", Unevaluated[x]]; x, HoldFirst];

In[2]:= 2 + 2

During evaluation of In[2]:= In[2] is: 2+2

Out[2]= 4

In[3]:= InString[2]

During evaluation of In[3]:= In[3] is: InString[2]

Out[3]= "\\(2 + 2\\)"

UPDATE 2

Replace $Pre with $PreRead in my code above and you get close to what you want, I believe:

In[1]:= $PreRead = Function[{x}, Print[Names["`*"]]; x, HoldFirst]

Out[1]= Function[{x}, Print[Names["`*"]]; x, HoldFirst]

In[2]:= a = 1

During evaluation of In[2]:= {x}

Out[2]= 1

In[3]:= b = 2

During evaluation of In[3]:= {a,x}

Out[3]= 2

It’s not possible to intercept In at the *Value level because the Kernel is simply not interacting with In via value manipulation in “top-level” Mathematica code.