10.2. Types of Errors

Each type of error—syntax, runtime, and logic—shows itself in a different way. Also, some strategies may be more useful for certain types of errors compared to others.

10.2.1. Stages for Running a Python Program

In order to understand programming errors, we should understand that code gets executed in two stages.

10.2.1.1. Stage 1 - Parsing

Before code can be run, it must be validated and prepared for execution. This is known as the parsing stage. Think of it like the pre-flight checklist pilots use before taking off. If errors are found, the plane stays on the ground.

When we click Run to start our code, parsing begins. A lot of detailed tasks happen behind the scenes during this process, but we just need to know that parsing checks the syntax and structure of the code. If any mistakes are found, the program does not launch.

10.2.1.2. Stage 2 - Execution

If parsing finishes without finding any errors, the program is ready to run. During the execution stage, the statements written into our code—printing to the console, prompting the user for input, making calculations, etc.—get carried out. Think of this stage as the plane taking flight.

10.2.2. Syntax Errors

Python can only run a program if the code is syntactically correct. Syntax refers to the structure of a language (spoken, programming, or otherwise) and the rules about that structure. For example, in English, a sentence must begin with a capital letter and end with some type of punctuation mark.

A syntax error occurs when we break one of the rules for a given language.

Examples

this sentence contains a syntax error.

So does this one

Here is a Python syntax error:

printt("Hello, World!")

For most readers, a few syntax errors are not a big problem. Our brains are flexible enough to figure out a sentence even if it contains one or more syntax errors.

Programming languages are not so forgiving. If there is a single syntax error anywhere in our program, Python will display an error message and immediately quit. Since syntax is checked during the parsing stage, these errors are found first.

As you completed the coding exercises in the last several chapters, you probably spent some time tracking down syntax errors. As you gain experience, you will make fewer of these mistakes, but they will never disappear completely. However, you will get good at quickly finding syntax errors.

Try It!

Find the syntax errors in the program.

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# Run the code to generate an error message.
# Find and fix only ONE syntax error at a time.

day = "Wednesday'
print(day
  1. What syntax errors did you find?
  2. What was the error message provided by Python in each case?
  3. Pay attention to any line numbers given in the error message.

10.2.3. Runtime Errors

Once the syntax for your code is correct, the program launches. However, mistakes can still occur and cause the program to crash. These mistakes are called runtime errors because they do not appear until we run the program. The errors are also called exceptions because they usually indicate that something exceptional (and bad) has happened.

Runtime errors occur during the execution phase of a program, so we only see them after all syntax mistakes are fixed.

Common runtime errors include:

  1. Misspelling a variable name.
  2. Using an index value that is too large for a given string or list.
  3. Incorrect math operations (like dividing by zero).
  4. Comparing different data types.

Examples

Run the following code to produce an error message. Read the message, fix the error, then re-run the program.

Continue this process until no more error messages appear. Fix only one error at a time.

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# Run the program to generate an error message.
# Fix only ONE error at a time, and re-run the program after each fix.

first_Name = "Jack"
word = 'Rutabaga'
number = 25

print(first_name)
print(word[25])
print(number/0)
print(number > word)

Make a note of name (e.g. IndexError) given to each of the runtime errors you find in the code. Also notice that the error message gives you the line number where the mistake occurred.

The syntax in the code is correct, but when the program runs, an error occurs that Python cannot solve. For example, on line 8, we try to print the value first_name, but the variable does not exist! (In line 4, we defined first_Name instead).

10.2.4. Logic Errors

The third type of error is the logic error (sometimes called a semantic error). If there is a logic error in our code, the program runs successfully and no error messages appear. However, the program does not work as intended.

The key characteristic of logic errors is that the program we wrote is not the program we wanted. The code runs just fine, but it does not solve the problem we need.

Let’s take a look at two examples of logic errors:

Examples

  1. We can use a conditional to tell us if an integer is even or odd:

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    num = 25
    
    if num%2 == 0:
       print("The number {0} is odd".format(num))
    else:
       print("The number {0} is even".format(num))
    

    Console Output

    The number 25 is even.
    
  2. To calculate our daily pay based on our weekly pay, we might try:

    Example

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    weekly_pay = 600
    
    daily_pay = weekly_pay / 7
    print(daily_pay)
    

    Console Output

    85.71428571428571
    

In the first example, the value 25 is odd, but our program calls it even. The syntax of the code is perfectly correct, and no runtime errors crash the program, but it gives us the wrong answer. The logic of the code is faulty. Since we know what the answer should be for 25, we can use that knowledge to help us correct our code. (There are several ways to fix the error—try to identify one option).

In the second example, the result surprises us because we thought we were making at least $100 per day (assuming we work Monday through Friday). According to the program, though, we only earn about $85 per day. The logic error occurs because we divided our weekly pay by 7 instead of the 5 days we actually came in to work.

Identifying logic errors can be tricky because unlike syntax and runtime problems, running the program produces no error messages that help us identify the issue. We must examine the output of the program and work backward to figure out what our code is doing wrong.

10.2.5. Check Your Understanding

Question

Label each of the following as either a syntax, runtime, or logic error.

Trying to use a variable that has not been defined.

  1. syntax error
  2. runtime error
  3. logic error

Leaving off a close parenthesis, ), when calling print.

  1. syntax error
  2. runtime error
  3. logic error

Using the formula seconds = minutes * 6 to calculate the number of seconds from a number of minutes.

  1. syntax error
  2. runtime error
  3. logic error

Question

Look back at the first logic error example. Which of the following would NOT correct the error?

  1. In line 3, use if num%2 != 0:
  2. In line 3, use if num%2 == 1:
  3. Switch the odd/even words in the print statements.
  4. In line 1, use num = 26