Common Python Errors and Solutions¶

When Python encounters a situation it cannot execute, it terminates the script and prints a Traceback. Understanding how to interpret tracebacks makes debugging straightforward.

How to Read a Python Traceback¶

A Python traceback should be read from the bottom to the top:

Traceback (most recent call last):
  File "calculator.py", line 18, in <module>
    result = compute(user_input)
  File "calculator.py", line 8, in compute
    return 100 / value
ZeroDivisionError: division by zero

1. Bottom line: States the exact exception type (ZeroDivisionError) and the descriptive error message (division by zero).
2. Line above the bottom: Indicates the exact file name (calculator.py) and line number (line 8) where the exception occurred.

1. SyntaxError: Invalid Syntax¶

Cause¶

Python encountered code that violates language grammar rules before execution began.

# Incorrect: Missing colon after if statement
# if score > 50
#     print("Pass")

# Correct:
if score > 50:
    print("Pass")

2. IndentationError: Expected an Indented Block¶

Cause¶

Python expects an indented block after statements ending in a colon (:) such as def, if, for, while, or class.

# Incorrect:
# def greet():
# print("Hello")

# Correct:
def greet():
    print("Hello")

3. NameError: Name is Not Defined¶

Cause¶

A variable or function is referenced that has not been defined in the current scope, or its name was misspelled.

user_name = "Alice"

# Incorrect (spelling mistake):
# print(username)  # NameError: name 'username' is not defined

# Correct:
print(user_name)

4. TypeError: Incompatible Types or Arguments¶

Cause¶

An operation or function was applied to an object of an inappropriate type.

age = 25

# Incorrect:
# message = "Age: " + age  # TypeError: can only concatenate str to str

# Correct: Use f-strings or str()
message = f"Age: {age}"

5. IndexError: List Index Out of Range¶

Cause¶

An index position was requested that is greater than or equal to the length of the sequence.

items = ["apple", "banana"]

# Incorrect: Indices are 0 and 1
# print(items[2])  # IndexError: list index out of range

# Correct:
if len(items) > 2:
    print(items[2])

6. KeyError: Key Not Found¶

Cause¶

A dictionary key was accessed using square brackets dict[key] that does not exist.

user = {"id": 1, "username": "alice"}

# Incorrect:
# email = user["email"]  # KeyError: 'email'

# Correct: Use .get() with a default value
email = user.get("email", "Not Provided")
print(email)

7. AttributeError: Object Has No Attribute¶

Cause¶

A method or attribute was accessed that does not exist on that data type.

text = "hello"

# Incorrect: Strings do not have an append() method
# text.append(" world") # AttributeError: 'str' object has no attribute 'append'

# Correct: Use concatenation (+)
text = text + " world"