Python - Bugs and Errors

Noted from Learn Python 3 | Codecademy

Introduction to Bugs

The story goes that on September 9th, 1947, computer scientist Grace Hopper found a moth in the Harvard Mark II computer’s logbook and reported the world’s first literal computer bug. However, the term “bug,” in the sense of technical error, dates back at least to 1878 and with Thomas Edison.

Python refers to the mistakes within the program as errors and will point to the location where an error occurred with a ^ character. When programs throw errors that we didn’t expect to encounter, we call those errors bugs. Programmers call the process of updating the program so that it no longer produces bugs debugging.

During your programming journey, we are destined to encounter innumerable red errors. Some even say that 75% of development time is spent on debugging. But what makes a programmer successful isn’t avoiding errors, it’s knowing how to solve them. And a good place to start is understanding what they are.

In Python, there are many different ways of classifying errors, but here are some common ones:

• SyntaxError: Error caused by not following the proper structure (syntax) of the language.
• NameError: Errors reported when the interpreter detects a variable that is unknown.
• TypeError: Errors thrown when an operation is applied to an object of an inappropriate type.

Syntax Errors

When we are writing Python programs, the interpreter is our first line of defense against errors.

SyntaxError means there is something wrong with the way your program is written — punctuation that does not belong, a command where it is not expected, or a missing parenthesis can all trigger a SyntaxError.

Here’s an example of a SyntaxError error message:

File "script.py", line 1
print(Hello world!)
^
SyntaxError: invalid syntax

The interpreter will tell us where (the file name and line number) it got into trouble and its best guess as to what is wrong.

After reading the error message, we can assume that the cause for this error is a lack of quotation marks around Hello world!.

Some common syntax errors are:

• Misspelling a Python keyword.
• Missing colon :.
• Missing closing parenthesis ), square bracket ], or curly brace }.

Name Errors

A NameError is reported by the Python interpreter when it detects a variable that is unknown.

This can occur if a variable is used before it has been assigned a value or if a variable name is spelled differently than the point at which it was defined. The Python interpreter will display the line of code where the NameError was detected and indicate which name is found that was not defined.

Here’s an example of a NameError error message:

File "script.py", line 1, in <module>
print(score)
NameError: name 'score' is not defined

This error is suggesting that the variable score was never defined in the program. Oops.

Some common name errors are:

• Misspelling a variable name.
• Forgetting to define a variable.

Type Errors

A TypeError is reported by the Python interpreter when an operation is applied to a variable of an inappropriate type.

This can occur in Python when one attempts to use an operator on something of the incorrect type.

For example, let’s see what happens when we try and add together two incompatible types:

piggy_bank = '2' + 0.25

There will be an TypeError error message:

Traceback (most recent call last):
File "script.py", line 1, in <module>
piggy_bank = '2' + 0.25
TypeError: must be str, not float

This error is reporting that 0.25 is not a string.

Some common type errors are:

• Accidentally adding or subtracting a string value.
• Call a function on something of the incorrect type.

There is also another type of error that doesn’t have error messages that we will cover down the line:

• Logic errors: Errors found by the programmer when the program isn’t doing what it is intending to do.

Remember, Google and Stack Overflow are a programmer’s BFFs (best friends forever) in situations where an error is giving you a lot of trouble. For some more motivation, check out this blog post.