Allow the %run magic with '-b' to specify a file.

[ellbur]

Hello,

Here is a minor tweak to the %run command that some users might find helpful:

The syntax would be:

%run -d -b file2.py:30 file1.py

to break at line 30 of file2.py when running file1.py.

The old syntax

%run -d -b 20 file1.py

still works the way it used to.

Suggested by user gozzilli on StackOverflow
(http://stackoverflow.com/questions/14305203/ipython-set-a-breakpoint-in-imported-file/).

[takluyver]

Should this one be adjusted? I'll just test.

[takluyver]

Hmm, I can't produce a situation where _exec_filename is actually used. Can you? If not, I think we shouldn't change it - it seems more likely that it is supposed to be the name of the top-level file that's being run, rather than the file the breakpoint is in.

[ellbur]

Yes, I think you're right. I didn't actually know what it was there for, so
I have no real reason to change it.

[takluyver]

The debugger uses it when looking for lines if the code was compiled with the filename <string>. But I don't know how to create a case where that happens.

Do you want to push another commit to the same branch reversing the change on this line?

[takluyver]

Oh, now I can get it, if I exec a string of code inside the module. I don't think there's a way of being certain about it in that case, but I still think we should leave it as filename for now.

[bfroehle]

Yes, this should be left as filename.

[bfroehle]

Other than the one change that @takluyver and I agree on, this is ready to go.

I looked through the pdb source hoping that there would a function we could reuse instead of parse_breakpoint, but came up empty. (The code is in pdb.Pdb.do_break and is significantly core complicated since it allows for breaking at functions...).

[bfroehle]

An alternative approach, which might have been simpler, would have been something more like:

bp_cmd = opts.get('b', [1])[0]
if ':' not in bp_cmd:
    bp_cmd = filename + ':' + bp_cmd
# ...
deb.do_break(bp_cmd)

But I'm not sure if deb.do_break would have run through all of the same checkline code that we do. As a side effect, this might have allowed breaking at function names (but I never implemented it to test it, so there might be other unexpected issues which would prevent that from working anyway).

For now I think the proposed patch here is plenty fine.

[ellbur]

OK, I changed bp_file back to filename. Thanks for looking at this :)

[takluyver]

OK, great. We'd normally want to add a test as well, especially as %run is one of the most delicate bits of the codebase. But I don't see an easy way to write an automated test for this.

@bfroehle has already OKed this as well, so I'm going to merge it now.