10 Common Python Mistakes and How to Fix Them

def append_item(item, lst=[]):
    lst.append(item)
    return lst

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

def append_item(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst

if x > 5
    print("x is large")

if x > 5:
    print("x is large")

if x = 5:  # SyntaxError
    print(x)

if x == 5:
    print(x)

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

nums = [1, 2, 3, 4, 5]
for num in nums:
    if num % 2 == 0:
        nums.remove(num)  # Skips elements!

nums = [1, 2, 3, 4, 5]
nums = [n for n in nums if n % 2 != 0]

if not value:  # Treats None, False, 0, "" the same
    print("empty")

if value is None:
    print("value is None")

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

my_list = [1, 2, 3]

lst = [1, 2, 3]
print(lst[5])  # IndexError

lst = [1, 2, 3]
if len(lst) > 5:
    print(lst[5])
# Or use get() for dicts

x = 10
def change_x():
    x = 20  # Creates local variable, doesn't change global
change_x()
print(x)  # 10

x = 10
def change_x():
    global x
    x = 20
change_x()
print(x)  # 20

result = ""
for i in range(1000):
    result += str(i)  # Slow! Creates new string each time

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