CODEWITHSUNDEEP
python
sashoalm
sashoalm

Reputation: 79467

Can I put a breakpoint in a running Python program that drops to the interactive terminal?

I'm not sure if what I'm asking is possible at all, but since python is an interpreter it might be. I'm trying to make changes in an open-source project but because there are no types in python it's difficult to know what the variables have as data and what they do. You can't just look up the documentation on the var's type since you can't be sure what type it is. I want to drop to the terminal so I can quickly examine the types of the variables and what they do by typing help(var) or print(var). I could do this by changing the code and then re-running the program each time but that would be much slower.

Let's say I have a program:

def foo():
    a = 5
    my_debug_shell()
    print a

foo()

my_debug_shell is the function I'm asking about. It would drop me to the '>>>' shell of the python interpreter where I can type help(a), and it would tell me that a is an integer. Then I type 'a=7', and some 'continue' command, and the program goes on to print 7, not 5, because I changed it.

Upvotes: 31

Views: 31270

Answers (7)

Nicolas Windt
Nicolas Windt

Reputation: 21

Within an active debugger session (e.g. your programm has already reached a breakpoint, wherever it might be) enter
b /path/to/my/awsome/programm.py:<line number>
This will set a break point at the given line.
For Example
b /path/to/my/awsome/programm.py:87
results in
Breakpoint 1 at /path/to/my/awsome/programm.py:87
Now enter c to continue the programme execution until the next breakpoint.
I presume this is the anwer to the question.

PS: I found it out by trying. I have found no hint about this anywhre on the web. Hope it helps.

Upvotes: 1

Kaka Ruto
Kaka Ruto

Reputation: 5125

Python 3.7+ has a new builtin way of setting breakpoints.

breakpoint()

The implementation of breakpoint() will import pdb and call pdb.set_trace().

Remember to include the braces (), since breakpoint is a function, not a keyword.

Upvotes: 13

Eric O. Lebigot
Eric O. Lebigot

Reputation: 94475

A one-line partial solution is simply to put 1/0 where you want the breakpoint: this will raise an exception, which will be caught by the debugger. Two advantages of this approach are:

  • Breakpoints set this way are robust against code modification (no dependence on a particular line number);

  • One does not need to import pdb in every program to be debugged; one can instead directly insert "breakpoints" where needed.

In order to catch the exception automatically, you can simply do python -m pdb prog.py… and then type c(ontinue) in order to start the program. When the 1/0 is reached, the program exits, but variables can be inspected as usual with the pdb debugger (p my_var). Now, this does not allow you to fix things and keep running the program. Instead you can try to fix the bug and run the program again.

If you want to use the powerful IPython shell, ipython -pdb prog.py… does the same thing, but leads to IPython's better debugger interface. Alternatively, you can do everything from within the IPython shell:

  • In IPython, set up the "debug on exception" mode of IPython (%pdb).
  • Run the program from IPython with %run prog.py…. When an exception occurs, the debugger is automatically activated and you can inspect variables, etc.

The advantage of this latter approach is that (1) the IPython shell is almost a must; and (2) once it is installed, debugging can easily be done through it (instead of directly through the pdb module). The full documentation is available on the IPython pages.

Upvotes: 10

ThomasH
ThomasH

Reputation: 23506

Here is a solution that doesn't require code changes:

python -m pdb prog.py <prog_args>
(pdb) b 3
Breakpoint 1 at prog.py:3
(pdb) c
...
(pdb) p a
5
(pdb) a=7
(pdb) ...

In short:

  • start your program under debugger control
  • set a break point at a given line of code
  • let the program run up to that point
  • you get an interactive prompt that let's you do what you want (type 'help' for all options)

Upvotes: 32

Matthew Schinckel
Matthew Schinckel

Reputation: 35619

You can run the program using pdb, and add breakpoints before starting execution.

In reality though, it's usually just as fast to edit the code and put in the set_trace() call, as another user stated.

Upvotes: 2

user2665694
user2665694

Reputation:

Not sure what the real question is. Python gives you the 'pdb' debugger (google yourself) and in addition you can add logging and debug output as needed.

Upvotes: 0

Amber
Amber

Reputation: 526573

http://docs.python.org/library/pdb.html

import pdb
pdb.set_trace()

Upvotes: 42

Related Questions