Artwork by @allison_horst

🐛 Debugging scenarios and solutions

Quick debugging tips

1. Read the error message and look at the line that it refers to.
   1. Sometimes the issue is at the line just above the line that was listed in the error.
   2. If enabled, pay close attention to the syntax highlighting in your editor, which can point at where the error might be happening.
2. Use print() statements to display the type and value of your variable.
3. Go back to the instructions and carefully re-read them.
4. Use Python Tutor to visualize your code.
5. Use the Rubber Duck Debugging to walk yourself through the logic of your code.
6. If you are having trouble with your output not matching what is expected, use an online text comparison tool, e.g. https://text-compare.com or https://contenttool.io/text-difference-checker.

Table of contents

1. Quick debugging tips
2. EOF (end-of-file) Errors
   1. EOFError: EOF when reading a line
   2. SyntaxError: unexpected EOF while parsing
3. EOL while scanning string literal
4. Indentation Errors
   1. IndentationError: unexpected indent
   2. IndentationError: expected an indented block
5. KeyError: ... with a dictionary
6. Name Errors
   1. NameError: name '...' is not defined
   2. NameError with a dictionary
7. Positional Arguments Errors
8. Syntax Errors
   1. SyntaxError: invalid syntax
   2. SyntaxError: unmatched ')'
9. Type Errors
   1. TypeError: argument of type 'int' is not iterable
   2. TypeError: argument of type 'NoneType' is not iterable
   3. TypeError: can only concatenate str (not "int") to str
   4. ValueError: invalid literal for int() with base 10
      1. TypeError: object of type '...' has no len()
      2. TypeError: ... takes exactly one argument (... given)
      3. TypeError: unsupported operand type(s) for +: 'int' and 'list'
10. Undesirable Results
    1. Output printing None
    2. Logic Errors
    3. Function address is printed <function function_name at 0x....>
11. Common autograder error messages on Gradescope
    1. Your submission timed out. It took longer than 600 seconds to run.
    2. Test Failed: Check the name of your .py file: it is incorrect.
    3. Test Failed: Syntax error when importing ...
12. Template
    1. Error: ...

EOF (end-of-file) Errors

EOFError: EOF when reading a line

• Cause:
  • Python is expecting an input but there is no input to provide.
  • Alternatively, there might be a missing closing parenthesis (see SyntaxError: unexpected EOF while parsing).
  • If the error is produced when using a loop, check that you do not have an infinite loop. Look closely at the loop’s conditions and verify that the loop stops, especially, in the edge cases.
  • In almost all cases, input() should be included inside the main program, not inside the functions.
• The program instructions might be providing 1 input value, but you have an extra input() in your code.

  num_items = int(input())
  item = input()
  

  but the input is just a single number, there’s no item to provide via input().

• An infinite loop example - num_items doesn’t get updated within the loop, so the condition stays always True (unless num_items == 0 and the loop is not executed):

  num_items = int(input())
  items = []
  while num_items:
    item = input("Enter an item: ")
    items.append(item)
  

SyntaxError: unexpected EOF while parsing

• Cause: Missing a closing parenthesis ).
• Example erroneous code:

  print('Hello, World'
  

• Corrected line of code:

  print("Hello, World")
  

EOL while scanning string literal

• Cause: Missing a closing quotation mark or a mismatched quotation mark (incorrectly placed quotation can also cause it).
• Example erroneous code:

  print('Hello, World) # missing a quotation mark
  

  or

  print('Hello, World") # mismatched quotation marks
  

Indentation Errors

IndentationError: unexpected indent

• Example erroneous code:

  print("Hello world!")
  print("What a nice day!")
  

• Cause: The code is either indented even though it is not necessary or is missing an indentation. The line number in the error points at where the mismatch happened.
• Correct code:

  print("Hello world!")
  print("What a nice day!")
  

IndentationError: expected an indented block

• Example erroneous code:

  def print_hello():
  """Print a simple greeting."""
  print("Hello!")
  

• Cause: This error occurs if the previous line ended with a colon : and did not have a properly indented line of code underneath it. It is likely that the issue occured at the line directly above the line reported in the error.
• Correct code (notice the indentation after the line that ends with a colon):

  def print_hello():
  """Print a simple greeting."""
  print("Hello!")
  

KeyError: ... with a dictionary

This error occurs when working with dictionaries. The line above the error shows which line caused the incorrect retrieval.

• Example erroneous code:

month_names = {
    "1": "January",
    "2": "February",
}
print("First month is", month_names[1]) # KeyError: 1

Alternatively, you wouldn’t get an error but would get an undesired output that displays None if using .get():

print("First month is", month_names.get(1)) # results in "First month is None"

• Cause: This error occurrs when trying to retrieve a value using a key that doesn’t exist in the dictionary. Possible causes:
  • the dictionary does not store the correct key-value pairs (did you add all necessary items? what about the edge cases?)
  • the key is stored / retrieved as an incorrect type (see the example)
• Correct code (alternative options):

...
print("First month is", month_names["1"])
print("First month is", month_names.get("1"))
print("First month is", month_names[str(1)])

Name Errors

NameError: name '...' is not defined

Python would replace the ellipsis ... with the name of the variable that it’s unable to find.

• Cause: Generally, this error happens when a function name or a variable name
  • is misspelled
  • has not been properly defined first

For example:

NameError: name 'Print' is not defined

• Example erroneous code:

  Print('Hello, World')
  

• Cause: In this case, the function name is “misspelled” - an accidentally capitalized name of the function makes Python look for a variable called Print.

This error can also be caused if there is a single word (or comma-separated words) inside the print() that needed to be displayed as the literal text but does not include quotation marks around it (similar to the SyntaxError: invalid syntax).

• Example erroneous code:

  print(Hello)
  print(Hello, World)
  

  generates the same error (NameError: name 'Hello' is not defined), since Python is now looking for a variable called Hello - it doesn’t know that we just forgot to put quotation marks around the text.

NameError with a dictionary

• NameError: name 'A' is not defined
• Example erroneous code:

  dict1 = {"A":1, "B":2, "C":3}
  print(dict1[A])
  

• Cause: Missing quotation marks (" ") when indexing a key from the dictionary.

• Correct code:

  dict1 = {"A":1, "B":2, "C":3}
  print(dict1["A"])
  

Positional Arguments Errors

Let’s first look at the case where too many arguments were provided in the function call. In that case, the error would be something like: print_name takes 0 positional arguments but 1 was given.

• Example erroneous code:

def print_name():
    print("Sam")

if __name__ == '__main__':
    print_name("Sam")

• Cause: The function print_name() does not take any parameters but when calling the function one parameter is being passed. This error signifies that there is a mismatch between the number of parameteres in the function defintion and the number of arguments in the function call.

• Correct code:

def print_name():
    print("Sam")

if __name__ == '__main__':
    print_name()

This also works the other way around if you are missing arguments in the function call. For example, the function below results in an error: get_largest() missing 1 required positional argument: 'y'.

• Example erroneous code:

def get_largest(x, y):
    return max(x, y)

if __name__ == '__main__':
    x = int(input())
    y = int(input())
    print(get_largest((x, y)))

• Cause: The function get_largest() takes in two parameters but when calling the function only one is passed in (i.e., a tuple with two elements). This error likewise signifies that there is a mismatch between the number of parameteres in the function defintion and the number of arguments in the function call.
• Correct code:

def get_largest(x, y):
    return max(x, y)

if __name__ == '__main__':
    x = int(input())
    y = int(input())
    print(get_largest(x, y))
  

Syntax Errors

SyntaxError: invalid syntax

This is a general error that occurs when the syntax is not correct and a “Python sentence” is broken. If enabled, pay close attention to the syntax highlighting, which can point at where the error might be happening.

Below are sample Python syntax rules:

1. A function call must start and end with a parenthesis; the number of open and closed parentheses must match.
2. The literal text has to be enclosed within the quotation marks.
3. Variables must first be defined (created) before being used.

• Cause: Missing opening quotation mark or opening parenthesis or quotations around the text.
• Example erroneous code:

  print(Hello World')
  print'Hello World  
  print(Hello World) # see also NameError
  

SyntaxError: unmatched ')'

• Cause: an extra closing parenthesis ) that does not have a matching opening paren (.
• Example erroneous code:

  print('Hello, World'))
  

Type Errors

TypeError: argument of type 'int' is not iterable

• Example erroneous code:

total = 42
sum(total)

• Cause: The error usually occurs when a built-in Python function that is intended for a sequence/collection is applied to an integer instead.
• Correct code:

total = [42]
sum(total)

TypeError: argument of type 'NoneType' is not iterable

• Example erroneous code:

val = None
if "a" in val:
    print("Found it!")

• Cause: The error usually occurs when the in operator in trying to index a None value instead of the sequence/collection.
  • Check the type/value of the object that is used after the in operator - if that object is a result of the function’s return value, verify that the function is returning the correct object or that your if branches are set up correctly to not try to index a None.
  • Do not store the result of print() and attempt to index it. Just like the methods that modify lists directly (since lists are mutable), print() does not return anything other than None.
• Correct code:

  val = None
  if val != None:
    if "a" in val:
        print("Found it!")
  

  or

  val = "aeou" # correct object provided
  if "a" in val:
    print("Found it!")
  

TypeError: can only concatenate str (not "int") to str

• Example erroneous code:

  num = 6
  print("I would like " + num + " tacos please.")
  

• Cause: You can only concatenate a string with a string, not a numeric type. Check the types of the variables that you are using in the concatenation.
• Correct code and alternatives:

  num = 6
  print("I would like " + str(num) + " tacos please.") # proper string concatenation
  print("I would like", num, "tacos please.") # using print defaults
  print(f"I would like {num} tacos please.") # using f-strings
  

ValueError: invalid literal for int() with base 10

• Example erroneous code:

  current_year = '1792.99'
  current_year = int(current_year)
  print(current_year)
  

• Cause: Float, represented as a string, cannot be directly converted into an integer. If you do want to pass a string representation of a float to an int, you need to convert to a float first, then to an integer.

• Correct code:

  current_year = '1792.99'
  current_year = float(current_year)
  current_year = int(current_year)
  print(current_year)
  

TypeError: object of type '...' has no len()

Examples include the errors

TypeError: object of type 'int' has no len()
TypeError: object of type 'float' has no len()

• Example erroneous code:

len(42)

• Cause: The error usually occurs when you think that you are asking for a length of a collection (e.g., a list, string, dictionary) but in reality the argument is an integer. Examine the value that is being provided as an argument into the len() function.

• Correct code:

  len([42]) 
  

TypeError: ... takes exactly one argument (... given)

Some examples of this error:

TypeError: len() takes exactly one argument (2 given)

• Example erroneous code:

len(42, 33)

• Cause: The error usually occurs when
  • a built-in Python function is being applied to an incorrect object.
  • the function is not given the required number of arguments.
• Correct code:

  len([42, 33]) 
  

  ---

TypeError: unsupported operand type(s) for +: 'int' and 'list'

• Example erroneous code:

nested_list = [[5, 10, 6], [7, 8, 9]]
total_sum = sum(nested_list)

• Cause: The error can occur when trying to sum up a nested list, instead of its individual elements.

• Correct code:

nested_list = [[5, 10, 6], [7, 8, 9]]
total_sum = 0
for item in nested_list:
    item_sum = sum(item)
    total_sum += item_sum

Undesirable Results

Output printing None

• None is printed even though you don’t want it to be there
• Example erroneous code:

def print_hello():
    print("hello")

if __name__ == '__main__':
    print(print_hello())

• Cause: The function print_hello() does not return anything (it has no return statement) so when you call print(print_hello()) you are printing its return value which is None.
• Correct code:

def print_hello():
    print("hello")

if __name__ == '__main__':
    print_hello()

Logic Errors

• Sometimes, we get a logic error, when the output does not match what we expect.
• Example erroneous code:

  def get_largest(x,y):
    if x > y: 
      return y
    else: 
      return x
  

• Cause: Although the syntax of this function is correct and the function will produce no errors, the result will be incorrect. This is simply due to a logical error - an incorrect value is being returned (or, the if condition needs to be changed).

• Correct code:

  def get_largest(x,y):
    if x > y: 
      return x
    else: 
      return y
  

Function address is printed <function function_name at 0x....>

• Function Address gets printed - <function function_name at 0x....>
• Example erroneous code:

def area(radius):
    return 3.14 * radius**2

if __name__ == '__main__':   
    radius = float(input())
    print(f"Circle has area {area} inches squared.")

• Cause: Instead of the function call, which requires the parentheses (and the arguments, if necessary), only the name of the function is used.
• Correct code:

  ...
  print(f"Circle has area {area(radius)} inches squared.")
  

Common autograder error messages on Gradescope

Your submission timed out. It took longer than 600 seconds to run.

• Cause: you have an infinite while loop in your code.
• Check: Carefully look at the condition in the while and ask when/if it will ever become False. Check when/where you are updating variables in the condition - the loop will keep going until the condition becomes False, so if the variable is not updated within the loop, you get an infinite loop.

Test Failed: Check the name of your .py file: it is incorrect.

• Cause: you did not name your file according to the instructions.
• We do not re-upload students’ files: if you would want to manually do something for all 280+ students in the class, please do not ask us to do it.

Test Failed: Syntax error when importing ...

• Cause: Something in your file is causing a syntax error (could be as simple as incorrect indentation; see the SyntaxError examples listed above).
• Check: Run your code and examine the line that is causing an error.

Template

Error: ...

This is a template for the error entries.

• Example erroneous code:

...

• Cause: …

• Correct code:

  ...
  

Acknowledgements

Developed by Yekaterina Kharitonova with assistance from students and course mentors.

Special thanks to Liu Kurafeeva for creating the initial formatting of this page.