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common_errors.md

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Original Reference (UC Berkeley CS61a Fall 2021): https://cs61a.org/articles/debugging/#common-bugs

Error Types

1. SyntaxError

Cause: code syntax mistake

Example:

  File "file name", line number
    def incorrect(f)
                    ^
SyntaxError: invalid syntax

Solution: the ^ symbol points to the code that contains invalid syntax. The error message doesn't tell you what is wrong, but it does tell you where. Notes: Python will check for SyntaxErrors before executing any code. This is different from other errors, which are only raised during runtime.

2. IndentationError

Cause: improper indentation

Example:

  File "file name", line number
    print('improper indentation')
IndentationError: unindent does not match any outer indentation level

Solution: The line that is improperly indented is displayed. Simply re-indent it. Notes: If you are inconsistent with tabs and spaces, Python will raise one of these. Make sure you use spaces! (It's just less of a headache in general in Python to use spaces and all cs61a content uses spaces).

3. TypeError

Cause 1:

  • Invalid operand types for primitive operators. You are probably trying to add/subract/multiply/divide incompatible types.
  • Example:
TypeError: unsupported operand type(s) for +: 'function' and 'int'

Cause 2:

  • Using non-function objects in function calls.
  • Example:
>>> square = 3
>>> square(3)
Traceback (most recent call last):
  ...
TypeError: 'int' object is not callable

Cause 3:

  • Passing an incorrect number of arguments to a function.
  • Example:
>>> add(3)
Traceback (most recent call last):
  ...
TypeError: add expected 2 arguments, got 1

4. NameError

Cause: variable not assigned to anything OR it doesn't exist. This includes function names.

Example:

File "file name", line number
  y = x + 3
NameError: global name 'x' is not defined

Solution: Make sure you are initializing the variable (i.e. assigning the variable to a value) before you use it. Notes: The reason the error message says "global name" is because Python will start searching for the variable from a function's local frame. If the variable is not found there, Python will keep searching the parent frames until it reaches the global frame. If it still can't find the variable, Python raises the error.

5. IndexError

Cause: trying to index a sequence (e.g. a tuple, list, string) with a number that exceeds the size of the sequence.

Example:

File "file name", line number
  x[100]
IndexError: tuple index out of range

Solution: Make sure the index is within the bounds of the sequence. If you're using a variable as an index (e.g. seq[x], make sure the variable is assigned to a proper index.

Common Bugs

1. Spelling

Python is case sensitive. The variable hello is not the same as Hello or hello or helo. This will usually show up as a NameError, but sometimes misspelled variables will actually have been defined. In that case, it can be difficult to find errors, and it is never gratifying to discover it's just a spelling mistake.

##2. Missing Parentheses A common bug is to leave off the closing parenthesis. This will show up as a SyntaxError. Consider the following code:

def fun():
    return foo(bar()   # missing a parenthesis here

fun()

Python will raise a SyntaxError, but will point to the line after the missing parenthesis:

File "file name", line "number"
    fun()
      ^
SyntaxError: invalid syntax

In general, if Python points a SyntaxError to a seemingly correct line, you are probably forgetting a parenthesis somewhere.

3. Missing close quotes

This is similar to the previous bug, but much easier to catch. Python will actually tell you the line that is missing the quote:

File "file name", line "number"
  return 'hi
           ^
SyntaxError: EOL while scanning string literal
EOL stands for "End of Line."

4. = vs. ==

The single equal sign = is used for assignment; the double equal sign == is used for testing equivalence. The most common error of this form is something like:

if x = 3:

5. Infinite Loops

Infinite loops are often caused by while loops whose conditions never change. For example:

i = 0
while i < 10:
    print(i)

Sometimes you might have incremented the wrong counter:

i, n = 0, 0
while i < 10:
    print(i)
    n += 1

6. Off-by-one errors

Sometimes a while loop or recursive function might stop one iteration too short. Here, it's best to walk through the iteration/recursion to see what number the loop stops at.