10 Common Python Mistakes and How to Fix Them

1. Mutable Default Arguments

```python

def append_item(item, lst=[]):

lst.append(item)

return lst

print(append_item(1)) # [1]

print(append_item(2)) # [1, 2] - Oops!

```

```python

def append_item(item, lst=None):

if lst is None:

lst = []

lst.append(item)

return lst

```

2. Forgetting Colons in Control Structures

```python

if x > 5

print("x is large")

```

```python

if x > 5:

print("x is large")

```

3. Confusion Between = and ==

```python

if x = 5: # SyntaxError

print(x)

```

```python

if x == 5:

print(x)

```

4. Off-by-One Errors with range()

```python

for i in range(1, 5): # Goes 1, 2, 3, 4 (not 5)

print(i)

```

5. Modifying Lists While Iterating

```python

nums = [1, 2, 3, 4, 5]

for num in nums:

if num % 2 == 0:

nums.remove(num) # Skips elements!

```

```python

nums = [1, 2, 3, 4, 5]

nums = [n for n in nums if n % 2 != 0]

```

6. Confusing None with False

```python

if not value: # Treats None, False, 0, "" the same

print("empty")

```

```python

if value is None:

print("value is None")

```

7. Shadowing Built-ins

```python

list = [1, 2, 3] # Now list() function is gone!

new_list = list() # TypeError

```

```python

my_list = [1, 2, 3]

```

8. Index Out of Range

```python

lst = [1, 2, 3]

print(lst[5]) # IndexError

```

```python

lst = [1, 2, 3]

if len(lst) > 5:

print(lst[5])

Or use get() for dicts

```

9. Forgetting Global Keyword

```python

x = 10

def change_x():

x = 20 # Creates local variable, doesn''t change global

change_x()

print(x) # 10

```

```python

x = 10

def change_x():

global x

x = 20

change_x()

print(x) # 20

```

10. String Concatenation in Loops

```python

result = ""

for i in range(1000):

result += str(i) # Slow! Creates new string each time

```

```python

result = "".join(str(i) for i in range(1000))

```

Summary

Avoid these mistakes by:

• Using linters (pylint, flake8)
• Writing tests
• Code reviews
• Practice on PythonExecutor!