Christian Long's Blog

Jun 15, 2015

ipdb, the iPython debugger


Python comes with a very useful debugger called pdb. The iPython project provides an enhanced version of pdb called ipdb. The ipdb debugger provides the same standard debugging commands as pdb, and it includes some nice additional features such as tab completion.


Install ipdb from the Python Package Index.

pip install ipdb

On Windows, you have to install the pyreadline library as well

pip install pyreadline

Basic debugging

Let’s step through a very simple Python program. Add this to a file called

def main():
    Main function
    x = 34
    y = 3
    z = y/x

if __name__ == '__main__':

Run it, wrapped in the ipdb debugger.


If you are using Python 3, the debugger script is called ipdb3 instead of ipdb


The debugger starts up, and waits at the first line.

> /home/<module>()
----> 1 def main():
      2     """
      3     Main function

First, let’s get help on what commands are available. Type ? and press enter.

ipdb> ?

Documented commands (type help <topic>):
EOF    bt         cont      enable  jump  pdef    psource  run      unt
a      c          continue  exit    l     pdoc    q        s        until
alias  cl         d         h       list  pfile   quit     step     up
args   clear      debug     help    n     pinfo   r        tbreak   w
b      commands   disable   ignore  next  pinfo2  restart  u        whatis
break  condition  down      j       p     pp      return   unalias  where

Miscellaneous help topics:
exec  pdb

Undocumented commands:
retval  rv

More help is available for each command.

ipdb> ?s
Execute the current line, stop at the first possible occasion
(either in a function that is called or in the current function).

Basic usage

Step through the program with s until you get past the line where 34 is assigned to x.

Now that we’re here, let’s see what value x holds. Type x and ipdb will display the value of x.

Set a breakpoint

ipdb> b 8
Breakpoint 1 at /home/

Use c to continue execution until that breakpoint.

Use q to quit out of the debugger.

iPython additions

ipdb adds some useful features above what plain pdb provides. The most useful is tab-completion. Any identifiers that are defined in locals() or in globals() can be tab-completed.

The “magic” commands (%, ??) from the regular iPython shell are not available in ipdb. Instead, ipdb provides some extra debugger commands, prefixed with a “p”. pinfo obj is the same as obj? in the iPython shell.

Step in to, past the definition of main(). Type pinfo main

ipdb> pinfo main
Signature: main()
Docstring: Main function
File:      ~/
Type:      function

This shows us some useful information about the main() function.

psource will display the source of an object.

ipdb> psource main
def main():
    Main function
    x = 34
    y = 3
    z = y/x

Further reading

We have just scratched the surface of pdb and ipdb. There is good information available right in the debugger, available by typing ?pdb.

For more information about plain pdb and its commands, the “Python Module of the Week” series did a nice introduction.


The pudb debugger presents a more visual display, with separate panes for breakpoints and local variables. It is available on Linux and OS X.

For more intractable problems, the Linux strace utility traces all system calls made by a program. It can attach to an already-running process, and can capture all the file and network input and output.

strace -o output.txt python
grep example output.txt

This shows us all the occurrences of the word ‘example’ in the system calls that happen when we run python

execve("/home/.virtualenvs/test/bin/python", ["python", ""], [/* 36 vars */]) = 0
readlink("", 0x7ffe244a2fe0, 4096) = -1 EINVAL (Invalid argument)
lstat("/home/", {st_mode=S_IFREG|0664, st_size=135, ...}) = 0
stat("", {st_mode=S_IFREG|0664, st_size=135, ...}) = 0
open("", O_RDONLY)            = 3
stat("", {st_mode=S_IFREG|0664, st_size=135, ...}) = 0
open("", O_RDONLY)            = 3

Let’s remove read permissions for

chmod a-r

If we run our strace again, we see that the open syscall now returns an error.

execve("/home/virtualenvs/test/bin/python", ["python", ""], [/* 36 vars */]) = 0
readlink("", 0x7ffc15852b60, 4096) = -1 EINVAL (Invalid argument)
lstat("/home/", {st_mode=S_IFREG|0220, st_size=135, ...}) = 0
stat("", {st_mode=S_IFREG|0220, st_size=135, ...}) = 0
open("", O_RDONLY)            = -1 EACCES (Permission denied)
stat("", {st_mode=S_IFREG|0220, st_size=135, ...}) = 0
open("", O_RDONLY)            = -1 EACCES (Permission denied)
write(2, "python: can't open file 'example"..., 67) = 67

Chad Fowler and Julia Evans have written good introductions to strace.