Common Python Errors and How to Fix Them

Beginners often encounter runtime and syntax errors when writing Python code; this guide lists 17 common mistakes, explains the resulting exceptions, and shows how to correct them.

1. Missing colon after statements (if, for, def, etc.) causes SyntaxError: invalid syntax . Example:

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

Fix: add a colon at the end of the statement.

2. Using assignment operator (=) instead of equality operator (==) also raises SyntaxError: invalid syntax . Example:

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

Fix: replace = with == for comparisons.

3. Incorrect indentation leads to IndentationError: unexpected indent , IndentationError: unindent does not match any outer indentation level , or IndentationError: expected an indented block . Example:

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

Fix: ensure indentation only follows lines ending with a colon and matches previous block levels.

4. Forgetting to call len() in a for loop results in TypeError: 'list' object cannot be interpreted as an integer . Example:

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

Fix: use range(len(spam)) or iterate directly over the list.

5. Trying to modify a string raises TypeError: 'str' object does not support item assignment . Example:

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

Fix: create a new string using slicing: <code>spam = "I have a pet cat." spam = spam[:13] + 'r' + spam[14:] print(spam)</code>

6. Concatenating non‑string values with strings triggers TypeError: Can't convert 'int' object to str implicitly . Example:

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

Fix: convert the integer to a string or use formatted output: <code>print('I have ' + str(numEggs) + ' eggs.') print('I have %s eggs.' % numEggs)</code>

7. Missing quotes around string literals causes SyntaxError: EOL while scanning string literal . Example:

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

Fix: ensure all string literals are enclosed in matching quotes.

8. Misspelling variable or function names leads to NameError: name 'fooba' is not defined . Example:

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

Fix: use the correct identifier spelling.

9. Misspelling method names raises AttributeError: 'str' object has no attribute 'lowerr' . Example:

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

Fix: use the correct method name, e.g., lower() .

10. Accessing a list index beyond its range triggers IndexError: list index out of range . Example:

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

Fix: ensure the index is within the list bounds.

11. Using a non‑existent dictionary key raises KeyError: 'spam' . Example:

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

Fix: check that the key exists or use dict.get() .

12. Using Python keywords as variable names results in SyntaxError: invalid syntax . Example:

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

Fix: avoid reserved words; see the list of Python keywords.

13. Using the increment operator ( += ) on an uninitialized variable causes NameError: name 'foobar' is not defined . Example:

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

Fix: initialize the variable before applying += .

14. Referencing a local variable before assignment when a global variable of the same name exists raises UnboundLocalError . Example:

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

Fix: declare the variable as global inside the function or avoid name clashes.

15. Trying to assign to a range() object yields TypeError: 'range' object does not support item assignment . Example:

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

Fix: convert the range to a list first: spam = list(range(10)) .

16. Using ++ or -- operators causes SyntaxError: invalid syntax . Example:

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

Fix: use spam += 1 or spam = spam + 1 .

17. Forgetting to include self as the first parameter of a class method results in TypeError: myMethod() takes no arguments (1 given) . Example:

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

Fix: define the method as def myMethod(self): .

These examples illustrate typical pitfalls for new Python developers and provide concrete corrections to improve code reliability.