I want hide all outputs created by Print command from user's function defined in module .m.

More precisely the problem is following:

I have a module that solves some specific task (It's not necessary to know the task for understanding the question). I use Print command inside the module to output process of evaluation. It's useful for debugging but not for using (all inner output must be hidden when user calls a function declared in the module).

So, in every stable version of the module I have to put comments (* *) and delete them for gebugging. I suppose that this way is uncivilized, and Mathematica should have a regular way to hide inner outputs.

For example, we have 3 files in the same directory. Can you suggest me a simple way how to hide all outputs by default and print all outputs in debugging mode?


f[x_]:=Module[{y}, y=Cos[x]*Sin[x];
Print["modul.m: y=",y];


<< modul.m;
f[x] (* print all inner output generated while f is evaluating*)
f[x](* print all inner output generated while f is evaluating*)


<< modul.m
(* Hide all outputs of f during evaluation. Command ; doesn't work for this issue. *) 

The short answer to this is that a simple Print[] is not a good way to debug a large package. It is not possible to turn it off (in a robust way) only for your package, but not in other places (it can be disabled globally, the simplest way being Print; Unprotect[Print]; Print = Null &, but this will severely annoy your users)

For much better ways to emit debug messages, and turn them off, see here:


I'll summarize Brett Champion and my own answers from there, as personally (and subjectively) I consider those the best solutions, but please read the full thread there.

Brett Champion's answer:

Use a custom symbol for debug messages(just like your package functions, you may want to put it in a separate context to avoid collisions), like this:


Then define a PrintDebug function like this:


PrintDebug[expr_] := Block[{debugPrint = Print}, expr]

Any code wrapped in PrintDebug will print messages. If there's no PrintDebug wrapper, message printing is disabled.

MyFunction[1,2,3] // PrintDebug

Alternately you may set a value to debugPrint globally to enable message printing for everything or just do $Pre = PrintDebug.

My answer:

We can use the built-in Message functionality for debugging too:

debugPrint::msg = "Debug message: `1`";    
debugPrint[msg_] := Message[debugPrint::msg, msg]

Use the function like this:


Turn off or on messages like this:


  • Thank you very much for your answer! I chose first solution (by Brett Champion's) since it can work with multiple argument in debugPrint debugPrint["modul.m: y=", y];debugPrint["modul.m: y=", y,"z=",z]; But the idea of using Message looks great. I wish there was posibility to define debugPrint with apriory unknown number of arguments. – Anonimous Jan 11 '12 at 7:37
  • @Anonimous When using the Message version of debugPrint, you can either put everything in a list before you pass it to it, debugPrint[{1,2,3}], or we could do something like debugPrint[args__] := Message[debugPrint::msg, StringJoin[ToString /@ {args}]] to emulate Print (unlike Print, this loses formatting, so you can't use it with e.g. Graphics!), or debugPrint[args__] := Message[debugPrint::msg, StringJoin[Riffle[ToString /@ {args}, ", "]]] to have comma separated arguments. Preserving formatting is also possible, but it's more advanced so I won't go into it. – Szabolcs Jan 11 '12 at 7:58
  • @Anonimous I just realized that StringForm is nestable, so you could use the message based debugPrint like this too: debugPrint[StringForm["x = ``, y = ``", x, y]] (or maybe integrate StringForm directly into the function) – Szabolcs Jan 11 '12 at 23:22

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