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Common beginner errors

Table of Contents

Overview

This chapter focuses on common mistakes beginners make in Python and how to fix them. You will see short examples of each error, what the error message usually looks like, and how to correct your code.

Use this chapter as a checklist when your program “mysteriously” doesn’t work.

Syntax and Typing Mistakes

These errors happen before your code even runs. Python stops and shows an error message.

Missing or extra colons `:`

Colons appear in several places, including:

Common mistake: 

if x > 10
    print("Too big")

Typical error:

Correct: 

if x > 10:
    print("Too big")

Similarly: 

for i in range(5)    # WRONG
    print(i)
while n > 0          # WRONG
    n = n - 1

Need colons: 

for i in range(5):
    print(i)
while n > 0:
    n = n - 1

Wrong indentation or inconsistent spaces

Python uses indentation to define blocks. Common mistakes:

Example: 

if age >= 18:
print("You are an adult")   # WRONG: not indented

Typical error:

Correct: 

if age >= 18:
    print("You are an adult")

Another common case: extra indentation where it’s not needed. 

    print("Hello")  # WRONG if not inside any block

Keep everything aligned consistently, and prefer 4 spaces per indent (no tabs).

Using `=` instead of `==` in conditions

= is assignment.
== is comparison.

Common mistake: 

if x = 5:          # WRONG
    print("x is 5")

Typical error:

Correct: 

if x == 5:
    print("x is 5")

Remember: only use = when storing a value, == when comparing values.

Unclosed quotes or parentheses

Forgetting to close:

Example: 

name = "Alice    # WRONG: no closing quote
print("Hello", name)

Or: 

print("Result is", (2 + 3   # WRONG: missing closing parenthesis

Typical errors:

Correct: 

name = "Alice"
print("Hello", name)
print("Result is", (2 + 3))

When you see these errors, carefully count your opening and closing characters.

Name and Variable Errors

These errors usually appear as NameError and often mean you typed something wrong or used it too early.

Misspelled variable or function names

Python is case-sensitive and spelling-sensitive.

Example: 

userName = "Alex"
print(username)   # WRONG: lowercase n

Typical error:

Correct: 

userName = "Alex"
print(userName)

Or, better, choose simple names and be consistent: 

username = "Alex"
print(username)

Same issue with built-in names: 

pritn("Hi")  # WRONG

Typical error:

Correct: 

print("Hi")

Using a variable before assigning it

Example: 

print(total)   # WRONG: total not defined yet
total = 10

Typical error:

Correct: 

total = 10
print(total)

Or inside conditions and loops: 

if done:
    print("Finished")   # WRONG if 'done' was never defined

Define variables before you use them.

Overwriting built-in names

Beginners sometimes use variable names that are already used by Python.

Example: 

print = "hello"       # Overwrites the built-in print function
print("Hi")           # Now this is a string, not a function

Typical error later:

Similar issues: 

list = [1, 2, 3]
dict = {}

These hide the built-in list and dict types.

Avoid using names like list, dict, str, int, input, print for your own variables.

Type-Related Mistakes

These errors involve mixing incompatible types.

Adding numbers and strings directly

User input is a string by default. Beginners often try: 

age = input("Enter your age: ")
years = 5
print("In 5 years you will be " + age + years)  # WRONG

Typical error:

Correct approaches:

  1. Convert the number to a string:
print("In 5 years you will be " + age + str(years))
  1. Or better, convert age to a number and use commas:
age = int(input("Enter your age: "))
years = 5
print("In 5 years you will be", age + years)

Related issue: 

result = "Answer: " + 10   # WRONG

Always check: are you combining str and int/float?

Forgetting to convert input to numbers

Example: 

x = input("Enter a number: ")
y = input("Enter another number: ")
print("Sum:", x + y)     # WRONG if you expect numeric sum

If x = "2" and y = "3", the output is:

Because strings are concatenated.

Correct: 

x = int(input("Enter a number: "))
y = int(input("Enter another number: "))
print("Sum:", x + y)

For decimals: 

x = float(input("Enter a decimal number: "))

Dividing by zero

Example: 

x = int(input("Enter a number: "))
print(10 / x)   # If x is 0

Typical error:

Avoid by checking the value: 

x = int(input("Enter a number: "))
if x != 0:
    print(10 / x)
else:
    print("Cannot divide by zero.")

Logic and Condition Mistakes

These errors don’t always stop the program, but they make it do the wrong thing.

Using `=` instead of comparison in conditions (logical mistake)

Sometimes this is caught as a syntax error (as shown before), but beginners may also mix up “assignment” and “comparison” conceptually.

Wrong idea: 

# "Set x to 5 in the if" – this is not how if works!
if x = 5:      # syntactically wrong AND logically wrong
    ...

Remember:

Wrong use of `and` / `or` / `not`

Example: age range check.

Common wrong attempts: 

age = 20
# WRONG: this does not do what most people expect
if age >= 18 or age <= 30:
    print("Age between 18 and 30")

With or, almost any age will satisfy one of the conditions.

Correct: 

if age >= 18 and age <= 30:
    print("Age between 18 and 30")

Or shorter: 

if 18 <= age <= 30:
    print("Age between 18 and 30")

Another mistake: placing not incorrectly. 

logged_in = False
# WRONG: double negative confusion
if not logged_in == True:
    print("Please log in")

This is confusing. Prefer: 

if not logged_in:
    print("Please log in")

Misusing `if` / `elif` / `else`

Common mistake: using separate if statements where elif is needed.

Example: 

score = 85
if score >= 90:
    grade = "A"
if score >= 80:
    grade = "B"   # OVERWRITES grade, even if it was "A"

Correct: 

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"

elif ensures only one block runs in that chain.

Another mistake: unreachable elif or else: 

if x > 10:
    ...
elif x > 20:     # This can never be true if x > 10 was already true
    ...

Think carefully about the order of your conditions.

Loop Mistakes

These often cause infinite loops or loops that run the wrong number of times.

Infinite `while` loops (condition never changes)

Example: 

count = 0
while count < 5:
    print(count)
    # Forgot to change count!

This prints 0 forever.

Correct: 

count = 0
while count < 5:
    print(count)
    count = count + 1

Always ask: “Does something inside the loop eventually make the condition false?”

Looping the wrong number of times with `range()`

Beginners often get confused by range(), especially off-by-one errors.

Example: 

# Want to print numbers 1 to 5
for i in range(1, 5):
    print(i)

This prints 1 2 3 4, because range(1, 5) stops before 5.

Correct: 

for i in range(1, 6):
    print(i)

Another mistake: assuming range(n) includes n. 

for i in range(5):
    print(i)

This prints 0 1 2 3 4, not 5.

Misusing `break` and `continue`

break exits the whole loop. continue skips to the next iteration.

Common mistake: putting break too early. 

for i in range(5):
    print(i)
    break      # Loop stops after first iteration

If you meant to stop only under a condition: 

for i in range(5):
    print(i)
    if i == 3:
        break

Or using continue in a way that accidentally creates an infinite loop in a while: 

count = 0
while count < 5:
    if count == 2:
        continue     # WRONG: count never increases when it's 2
    print(count)
    count += 1

Here, when count becomes 2, the program keeps hitting continue and never increments count.

Correct: change the order: 

count = 0
while count < 5:
    if count == 2:
        count += 1
        continue
    print(count)
    count += 1

Working with Collections (Lists, Strings, etc.)

Using the wrong index or going out of range

Every list and string in Python is zero-indexed:

Common mistake: 

numbers = [10, 20, 30]
print(numbers[3])   # WRONG, only indices 0,1,2 exist

Typical error:

Correct: 

print(numbers[0])  # 10
print(numbers[1])  # 20
print(numbers[2])  # 30

Similar issues when looping: 

names = ["Ana", "Bob", "Cara"]
for i in range(0, len(names) + 1):   # WRONG: +1 causes out of range
    print(names[i])

Correct: 

for i in range(0, len(names)):
    print(names[i])

Or simpler: 

for name in names:
    print(name)

Confusing list methods like `append` and `extend`

Common mistake: 

nums = [1, 2, 3]
nums = nums.append(4)   # WRONG
print(nums)

append changes the list in place and returns None, so nums becomes None.

Correct: 

nums = [1, 2, 3]
nums.append(4)
print(nums)   # [1, 2, 3, 4]

Another confusion: append vs extend. 

nums = [1, 2, 3]
nums.append([4, 5])
print(nums)   # [1, 2, 3, [4, 5]]  (a list inside a list)

If you wanted to add 4 and 5 as separate elements: 

nums = [1, 2, 3]
nums.extend([4, 5])
print(nums)   # [1, 2, 3, 4, 5]

Input/Output and Printing Mistakes

Forgetting parentheses with `print` in Python 3

If you have seen Python 2 examples online, you might write: 

print "Hello"   # WRONG in Python 3

Typical error:

Correct: 

print("Hello")

Printing the wrong thing (logic vs syntax)

Example: trying to print the result of a calculation but printing the expression as a string. 

x = 2
y = 3
print("x + y")   # Prints the text "x + y", not 5

Correct: 

print(x + y)           # 5
print("x + y =", x + y)  # x + y = 5

Function and Scope Mistakes

(Details of functions and scope appear in their own chapters; here we focus on typical beginner errors.)

Calling a function before defining it (in the wrong place)

Example in a script: 

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

Typical error:

Correct: 

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

Python executes top to bottom in a file; define functions before you call them (except in some advanced patterns).

Forgetting to use `return`

Example: 

def add(a, b):
    result = a + b    # WRONG: no return
# ...
sum_value = add(2, 3)
print(sum_value)

Typical result:

Correct: 

def add(a, b):
    result = a + b
    return result
sum_value = add(2, 3)
print(sum_value)   # 5

Or shorter: 

def add(a, b):
    return a + b

File and Path Mistakes (Basic)

(There is a full chapter on working with files; here are just a few beginner errors.)

Using the wrong file path or assuming the wrong folder

Common mistake: 

f = open("data.txt")

Typical error:

Reasons:

Quick checks:

Example with a relative path: 

f = open("files/data.txt")   # if data.txt is inside a "files" folder

Always close files (or use with), but the main beginner issue is wrong locations/name.

Misreading and Ignoring Error Messages

A very common beginner error is not reading the error message carefully at all.

Typical mistake:

Better approach:

  1. Look at the type of error (e.g. SyntaxError, NameError, TypeError).
  2. Look at the line number Python points to.
  3. Read the message in plain English.

Example: 

values = [1, 2, 3]
print(values[3])

Error: 

IndexError: list index out of range

This tells you:

Use error messages as hints, not enemies.

How to Practice Avoiding These Errors

Over time, these “beginner errors” become rare, and when they appear, you will know exactly where to look.

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