17 Common Python Errors Every Beginner Should Know

For beginners learning Python, encountering errors is inevitable. The following collection of 17 typical mistakes, their error messages, and example code helps you identify and resolve these problems efficiently.

1. Missing colon after statements

Forgetting the colon (:) at the end of if , for , def , elif , else , class , etc., triggers a SyntaxError: invalid syntax .

<code>if spam == 42
    print('Hello!')</code>

2. Using = instead of ==

Assigning rather than comparing also raises SyntaxError: invalid syntax .

<code>if spam = 42:
    print('Hello!')</code>

3. Incorrect indentation

Wrong indentation leads to various IndentationError messages such as "unexpected indent", "unindent does not match any outer indentation level", or "expected an indented block".

<code>print('Hello!')
    print('Howdy!')</code>

or

<code>if spam == 42:
    print('Hello!')
 print('Howdy!')</code>

4. Forgetting len() in a for loop

Using a list directly in range() causes TypeError: 'list' object cannot be interpreted as an integer .

<code>spam = ['cat', 'dog', 'mouse']
for i in range(spam):
    print(spam[i])</code>

5. Trying to modify a string

Strings are immutable; attempting item assignment raises TypeError: 'str' object does not support item assignment .

<code>spam = 'I have a pet cat.'
spam[13] = 'r'
print(spam)</code>

Correct approach:

<code>spam = 'I have a pet cat.'
spam = spam[:13] + 'r' + spam[14:]
print(spam)</code>

6. Concatenating non‑string values with strings

Adding an integer to a string raises TypeError: Can't convert 'int' object to str implicitly .

<code>numEggs = 12
print('I have ' + numEggs + ' eggs.')</code>

Fix by converting the integer:

<code>numEggs = 12
print('I have ' + str(numEggs) + ' eggs.')
print('I have %s eggs.' % numEggs)</code>

7. Missing quotes around string literals

Omitting quotes results in SyntaxError: EOL while scanning string literal .

<code>print(Hello!')
print('Hello!)
myName = 'Al'
print('My name is ' + myName + . How are you?')</code>

8. Misspelled variable or function name

This produces NameError: name 'fooba' is not defined .

<code>foobar = 'Al'
print('My name is ' + fooba)
spam = ruond(4.2)
spam = Round(4.2)</code>

9. Misspelled method name

Calling a non‑existent method, such as lowerr , raises AttributeError: 'str' object has no attribute 'lowerr' .

<code>spam = 'THIS IS IN LOWERCASE.'
spam = spam.lowerr()</code>

10. Index out of range

Accessing a list element beyond its length triggers IndexError: list index out of range .

<code>spam = ['cat', 'dog', 'mouse']
print(spam[6])</code>

11. Using a non‑existent dictionary key

Attempting to access a missing key raises KeyError: 'spam' .

<code>spam = {'cat': 'Zophie', 'dog': 'Basil', 'mouse': 'Whiskers'}
print('The name of my pet zebra is ' + spam['zebra'])</code>

12. Using a Python keyword as a variable name

This results in SyntaxError: invalid syntax .

<code>class = 'algebra'</code>

13. Using an increment operator on an uninitialized variable

Doing spam += 1 without a proper initial value raises NameError: name 'foobar' is not defined .

<code>spam = 0
spam += 42
eggs += 42</code>

14. Referencing a local variable before assignment

If a variable is assigned later in the function, using it earlier causes UnboundLocalError: local variable 'foobar' referenced before assignment .

<code>someVar = 42
def myFunction():
    print(someVar)
    someVar = 100
myFunction()</code>

15. Trying to assign to a range object

Since range() returns a range object, the following raises TypeError: 'range' object does not support item assignment .

<code>spam = range(10)
spam[4] = -1</code>

Correct way:

<code>spam = list(range(10))
spam[4] = -1</code>

16. Using ++ or -- operators

Python does not support these increment/decrement operators, leading to SyntaxError: invalid syntax .

<code>spam = 1
spam++</code>

Correct form:

<code>spam = 1
spam += 1</code>

17. Forgetting the self parameter in a method

Defining a class method without self causes TypeError: myMethod() takes no arguments (1 given) .

<code>class Foo():
    def myMethod():
        print('Hello!')
a = Foo()
a.myMethod()</code>

Include self as the first parameter to fix the error.