Finding and fixing errors in Python

Syntax error: the code breaks the rules of the language, so it cannot run at all. Examples: a missing colon, bracket or quote, or a misspelt keyword.
Runtime error: the code starts, but crashes while running. Examples: dividing by zero, using a list index that does not exist, or turning "ten" into an integer.
Logic error: the program runs and does not crash, but gives the wrong result. Python gives no message, so you find these by testing.

The last line of the message names the error. Look at the line number it gives, and also the line just before it: a missing bracket is often reported on the next line.

This program should count marks of 50 or more, but prints 2 instead of 4:
# Count the passes: a mark of 50 or more is a pass
marks = [72, 50, 38, 50, 91]
passes = 0
for mark in marks:
if mark > 50:
passes = passes + 1
print(passes)
The two marks of exactly 50 are missed because the condition uses > instead of >=. Testing with boundary data (exactly 50) finds this kind of error.
Common logic errors: the wrong comparison operator, a loop that runs one time too many or too few, a variable reset inside a loop, and a calculation done inside a loop instead of after it.

Read the error message and go to the line.
Use a trace table to follow the values of the variables.
Add temporary print() lines to show values while the program runs.
Test with normal, boundary and erroneous data, and compare the actual output with the expected output.