Debugging is a process that resolves the issues that occur in your code and prevent your software from running properly. In Python, debugging is very easy. The Python debugger sets conditional breakpoints and debugs the source code one line at a time. We'll debug our Python scripts using a pdb module that's present in the Python standard library.

To better debug a Python program, various techniques are available. We're going to look at four techniques for Python debugging:

• print() statement: This is the simplest way of knowing what's exactly happening so you can check what has been executed.
• logging: This is like a print statement but with more contextual...

In this section, we're going to learn how Python handles exceptions. But first, what is an exception? An exception is an error that occurs during program execution. Whenever any error occurs, Python generates an exception that will be handled using a try…except block. Some exceptions can't be handled by programs so they result in error messages. Now, we are going to see some exception examples.

In your Terminal, start the python3 interactive console and we will see some exception examples:

student@ubuntu:~$ python3
Python 3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> 50 / 0


Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ZeroDivisionError...

There are many debugging tools supported in Python:

• winpdb
• pydev
• pydb
• pdb
• gdb
• pyDebug

In this section, we are going to learn about pdb Python debugger. pdb module is a part of Python's standard library and is always available to use.

The pdb module is used to debug Python programs. Python programs use pdb interactive source code debugger to debug the programs. pdb sets breakpoints and inspects the stack frames, and lists the source code.

Now we will learn about how we can use the pdb debugger. There are three ways to use this debugger:

• Within an interpreter
• From a command line
• Within a Python script

We are going to create a pdb_example.py script and add the following content in that...

In this section, we are going to see the trace module. The trace module helps in tracing the program execution. So, whenever your Python program crashes, we can understand where it crashes. We can use trace module by importing it into your script as well as from the command line.

Now, we will create a script named trace_example.py and write the following content in the script:

class Student:
            def __init__(self, std):
                        self.count = std
 
            def go(self):
                        for i in range(self.count):
                                    print(i)
                        return
if __name__ == '__main__':
            Student(5).go()

The output will be as follows:

student@ubuntu:~$ python3 -m trace --trace trace_example.py
 --- modulename: trace_example, funcname: <module>
trace_example.py(1): class...

Profiling a Python program means measuring an execution time of a program. It measures the time spent in each function. Python's cProfile module is used for profiling a Python program.

As discussed previously, profiling means measuring the execution time of a program. We are going to use the cProfile Python module for profiling a program.

Now, we will write a cprof_example.py script and write the following code in it:

mul_value = 0
def mul_numbers( num1, num2 ):
            mul_value = num1 * num2;
            print ("Local Value: ", mul_value)
            return mul_value
mul_numbers( 58, 77 )
print ("Global Value: ", mul_value)

Run the program and you will...

There are various ways to make your Python programs run faster, such as the following:

• Profile your code so you can identify the bottlenecks
• Use built-in functions and libraries so the interpreter doesn't need to execute loops
• Avoid using globals as Python is very slow in accessing global variables
• Use existing packages

In this chapter, we learned about the importance of debugging and profiling programs. We learned what the different techniques available for debugging are. We learned about the pdb Python debugger and how to handle exceptions. We learned about how to use the cProfile and timeit modules of Python while profiling and timing our scripts. We also learned how to make your scripts run faster.

In the next chapter, we are going to learn about unit testing in Python. We are going to learn about creating and using unit tests.

1. To debug a program, which module is used?
2. Check how to use ipython along with all aliases and magic functions.
3. What is Global interpreted lock (GIL)?
4. What is the purpose of the PYTHONSTARTUP, PYTHONCASEOK, PYTHONHOME, and PYTHONSTARTUP environment variables?

5. What is the output of the following code? a) [0], b) [1], c) [1, 0], d) [0, 1].

def foo(k):
    k = [1]
q = [0]
foo(q)
print(q)

6. Which of the following is an invalid variable?
   a) my_string_1
   b) 1st_string
   c) foo
   d) _

• How to handle GIL problems in python: https://realpython.com/python-gil/
• Check how to use pdb module in command line: https://fedoramagazine.org/getting-started-python-debugger/