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When Python is executed externally, in Windows, a DOS-console is opened openned and it is possible to use the Python function print() to show values in this console:

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But when executing python code inside GiD, this console does not exists, and thus, this command cannot be used, except when using IDLE shell ( Utilities -> Tools -> Develop -> Tools → Console Python… ) .

Python print to file

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f=open('C:/temp/my_debug.txt','a')
f.write('hello world\n')
f.close()

Python

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show text with

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GiD’s proc W

If Python is running in GiD then the Tcl GiD procedure W can be called, which shows text in a window.
e.g.

GiD_Python_Exec {
  import tohil
  tcl=tohil.import_tcl()    ;# defines in python all defined Tcl proc's at this time
  a=5.2
  tcl.W(a)
  b=a*3
  tcl.W(b)
}

will Will show a GiD window with the value of the variables 'a' and 'b'

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Note: Important note:

Tcl follows a ’lazy' approach, this means that at a certain point in time of execution, only the procedures do exist that have been called or that have been defined from the current or previous loaded scripts.

tcl = tohil.import_tcl() is a tohil module command that uses Tcl’s introspection capabilities to create creates a Python object with methods for each Tcl proc and commandcommands, so that calling the Tcl procs looks very much like calling any Python function.
Warning: the Python methods created depends on the procs existing when tcl = tohil.import_tcl() is invoked. If a proc like W is already not defined in Tcl during this call, the Python method tcl.W('hello world') won’t exists, but tohil.call('W','hello world') will works.Warning: It seems that tcl.W() cannot uses Tcl’s introspection capabilities to map Tcl procs and commands to methods of the Python object. It will only map existing Tcl procs and commands at the time of invocation!
This means that the above code will fail if the Tcl proc W has never been called before.
To make sure the above code works, first execute -np- W "Hello World" in GiD’s command line and then execute the above code.

Using tohil.call('W','hello world') will work as it delegates to the internal tohil Tcl interpreter the execution of the string list parameters, instead of calling the Tcl-proc directly. This way the internal Tcl interpreter will look for the definition of the procedure W if it does not exists, and execute it with 'hello world' as parameter.

tcl.W() can also be called from the IDLE shell window or will crash

By now do not try something like this or GiD will crash:

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In this case it is less interesting to be used, because can use print(). The most interesting case is to use to debug code of GiD_Python_Exec, or GiD_Python_Sourceof GiD Utilities-->Tools-->Python console...:

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Warning: It seems that tcl’s command update cannot be called from the IDLE shell window, showing following message ( MS Windows and Linux):

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But if you click ‘OK’ the code will continue its execution.
Be aware that the command update is used in several GiD's windows and utilities, so this error message will appear several times.
Note: on macOS tcl’s command update will cause GiD likely to crash.

And in windows also crash, to avoid it we have modified the source code of Modules\_tkinter.c and recompiled.