Evaluate Sage code using the interactive tracer and return the result. The string code must be a valid expression enclosed in quotes (no assignments - the result of the expression is returned). In the Sage notebook this just raises a NotImplementedException.
INPUT:
REMARKS: This function is extremely powerful! For example, if you want to step through each line of execution of, e.g., factor(100), type
sage: trace("factor(100)") # not tested
then at the (Pdb) prompt type s (or step), then press return over and over to step through every line of Python that is called in the course of the above computation. Type ? at any time for help on how to use the debugger (e.g., l lists 11 lines around the current line; bt gives a back trace, etc.).
Setting a break point: If you have some code in a file and would like to drop into the debugger at a given point, put the following code at that point in the file:
import pdb; pdb.set_trace()
For an article on how to use the Python debugger, see http://www.onlamp.com/pub/a/python/2005/09/01/debugger.html
TESTS: The only real way to test this is via pexpect spawning a sage subprocess that uses IPython.
sage: import pexpect
sage: s = pexpect.spawn('sage')
sage: _ = s.sendline("trace('print factor(10)'); print 3+97")
sage: _ = s.sendline("s"); _ = s.sendline("c");
sage: _ = s.expect('100', timeout=90)
Seeing the ipdb prompt and the 2 * 5 in the output below is a strong indication that the trace command worked correctly.
sage: print s.before[s.before.find('--'):]
--...
ipdb> c
2 * 5
We test what happens in notebook embedded mode:
sage: sage.plot.plot.EMBEDDED_MODE = True
sage: trace('print factor(10)')
Traceback (most recent call last):
...
NotImplementedError: the trace command is not implemented in the Sage notebook; you must use the command line.