ipdb: Interactive Python Debugger with IPython

ipdb is an interactive Python Debugger with IPython integration, which features tab completion and syntax highlighting, etc. In layman's terms, ipdb is a better pdb.

ref:
https://github.com/gotcha/ipdb

Usage

$ pip install -U ipdb

ref:
https://pypi.python.org/pypi/ipdb

Add a breakpoint to any place you want to inspect, then run your code.

import ipdb; ipdb.set_trace()

If you use Sublime Text 3, try Python Breakpoints.
https://github.com/obormot/PythonBreakpoints

Useful Commands

Oldest frame is the frame in the stack where your program started; it is the oldest in time; the Newest frame, the other end of the stack, is where Python is executing code and is the current frame of execution.

# help: Print the list of all commands
h

# help: Print help about the certain command
h break

# print: Print the value of the expression
p some_obj
pp some_obj

# Print detailed information about the object
pinfo some_obj
pinfo2 some_obj

# args: Print arguments with their values of the current function
a

# list: List 11 lines of source code around the current line
l

# list: List 11 lines of source code around line 123
l 123

# longlist: List all source code for the current function or frame
ll

# jump: Jump to line 123, skip the execution of anything between
j 123

# args: List all arguments of the current function
a

# step: Execute code line by line, it may jump to another frame when a function call is encountered
s

# next: Execute code line by line, it doesn't enter functions called from the statement being executed
n

# return: Continue execution until the current function returns.
r

# continue: Continue execution, only stop when a breakpoint is encountered
c

# break: List all breakpoints
b

# break: Set a breakpoint at line 123
b 123

# break: Set a breakpoint at line 123 of file.py
b path/to/file.py:123

# break: Set a breakpoint on some_func that will be triggered if some_arg == 0
b some_func, some_arg == 0

# clear: Clear all breakpoints
clear

# where: Print a stack trace
w

# up: Move the current frame one level up in the stack trace
u

# down: Move the current frame one level down in the stack trace
d

# quit: Quit debugging
q

# use ! to run Python code that may conflict with pdb's built-in commands
!r = 123
!r = 123; c = 455

ref:
https://docs.python.org/2/library/pdb.html#debugger-commands
https://docs.python.org/3/library/pdb.html#debugger-commands
https://pymotw.com/2/pdb/
https://pymotw.com/3/pdb/
https://medium.com/instamojo-matters/become-a-pdb-power-user-e3fc4e2774b2

post_mortem

Debugging a failure after a program terminates is called post-mortem debugging.

>>> do_shit()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pdb_post_mortem.py", line 13, in go
    for i in range(self.num_loops):
AttributeError: 'MyObj' object has no attribute 'num_loops'
>>> import ipdb; ipdb.pm()

trace

Tracing a program as it runs. In this case, it will enter ipdb when sys.path changes.

import sys

# this function will execute on every line!!!
def trace_sys_path(frame, event, arg):
    if sys.path[0].endswith('/lib'):
        ipdb.set_trace()
    return trace_sys_path

sys.settrace(trace_sys_path)

ref:
https://youtu.be/5XvAVgcbmdY?t=22m51s

Use IPython magic functions in ipdb

Because that ipdb is not a full IPython shell: actually, it provides the same Python Debugger interface as pdb, ipdb lacks many features of IPython, for instance, magic functions. You could use following code to enter a real IPython environment for debugging.

from IPython import embed; embed()

Instead of import ipdb; ipdb.set_trace().

ref:
http://stackoverflow.com/questions/16184487/use-ipython-magic-functions-in-ipdb-shell
https://github.com/gotcha/ipdb/issues/33