How does PyROOT change the python interpreter?

Question

If I try to run .ls in python, not surprisingly I get a SyntaxError

>>> .ls
  File "<stdin>", line 1
    .ls
    ^
SyntaxError: invalid syntax

But if I import PyROOT, it somehow makes this syntax legal (and behaves as it does in ROOT, listing the contents of the current file; in the example here I haven't opened any.)

>>> import ROOT
>>> .ls
>>>

Similarly, .q works to quit the Python interpreter after I've imported ROOT, as it does in the normal ROOT interpreter.

How does this work?

Answer 1

It can by found in lib/ROOT.py

there is condition that if not ipython the sys.excepthook is redefined:

sys.excepthook = _excepthook

which in turn contains stuff like:

### RINT command emulation     
------------------------------------------------------
def _excepthook( exctype, value, traceb ):
 # catch syntax errors only (they contain the full line)
   if isinstance( value, SyntaxError ) and value.text:
      cmd, arg = split( value.text[:-1] )

    # mimic ROOT/CINT commands
      if cmd == '.q':
         sys.exit( 0 )

or few lines bellow:

  elif cmd == '.ls':
     return sys.modules[ __name__ ].gDirectory.ls()

and if non of those work it resumes normal handling of things.