ipdb.md

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###Debugger (i)pdb Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program. All of you have debugged some program or script in any moment of your developer's life, but probably not all of you have used a debugger.

A debugger is a tool that helps you explore the execution of your code in a very controlled way, allowing you to execute it line by line and change the behaviour "on the go".

Some of the reasons why you want to use a debugger are:

• Trying to spot a bug with print statements withing the code is hard, probably useless and chances are that you're gonna forget some of these statements when you "clean up" your code afterwards.
• It is practically useless to "debug" if you cannot interact with or change what is causing the bug (again, print is not helpful here).
• Check and change values of variables, methods, objects, etc. while debugging

pdb, or the IPython debugger ipdb allows you to debug your python code. To enter the debugger, you just have to insert a single statement in your code.

Let's look at it with an example, imagine that you have this function:

def sum_list(l):
    """Sum all elements in a list

    Arguments:
        l -- list to sum
    """
    return sum(l)

This method will crash if you pass it a list with mixed integers and strings, for example, but you don't know that because the function is inside a module, that is called by another function, that bla bla... What you'd do is to add the following statement:

def sum_list(l):
    """Sum all elements in a list

    Arguments:
        l -- list to sum
    """
    import ipdb; ipdb.set_trace()
    return sum(l)

The ipdb.set_trace() statement will print a trace so you know where you are.

When you run your method, ipdb will stop the execution where that statement is located and you'll be on an interactive shell, where you can do whatever you want.

For example, we could explore the variable l to see its contents:

With this small example you can see how useful can be ipdb. In this case would be enough to raise an exception with a proper message or try so sum only the int values.

Here it follows a list of the most common used ipdb commands. For a complete documentation, go here!

####(i)pdb useful commands

• h(elp) [command]: Without argument, print the list of available commands. With a command as argument, print help about that command.
• w(here): Print a stack trace, with the most recent frame at the bottom. An arrow indicates the current frame, which determines the context of most commands.
• s(tep): Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).
• n(ext): Continue execution until the next line in the current function is reached or it returns. (The difference between next and step is that step stops inside a called function, while next executes called functions at (nearly) full speed, only stopping at the next line in the current function.) *c(ontinue): Continue execution, only stop when a breakpoint is encountered. *a(rgs): Print the argument list of the current version.
• p expression: Evaluate the expression in the current context and print its value.
• q(uit): Quit from the debugger. The program being executed is aborted.

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