if / elif / else and Advanced Branching

The basics of if and if not

if 条件: runs the block only when the condition is True — it's the most basic branching construct. To negate a condition, prepend not, as in if not 条件:.

not is the operator that flips True and False, so phrases like "when it's not …" or "when it isn't empty" can be written naturally.

# if: runs only when the condition is True
is_logged_in = True
if is_logged_in:
    print("Welcome")
# Output: Welcome

# if not: flips the condition before evaluating
is_logged_in = False
if not is_logged_in:
    print("Please log in")
# Output: Please log in

# Combining with comparison expressions
cart = []
if not cart:
    print("Your cart is empty")
# Output: Your cart is empty

Check whether a user has verified their email.

① Define email_verified = False.

② Use if email_verified: to print "Thanks for using our service" when verified.

③ Use if not email_verified: to print "Please complete email verification" when not verified.

(The explanation appears once you run the code correctly.)

Writing multi-way branches with elif and else

To handle several branches, use elif and else.

elif is short for else if — when the if condition is False, Python tries the next condition, and if none of them match, the final else runs.

# Choose an instruction based on a traffic signal
signal = "YELLOW"

if signal == "GREEN":
    print("Go")
elif signal == "YELLOW":
    print("Proceed with caution")
elif signal == "RED":
    print("Stop")
else:
    print("Unknown signal")
# Output: Proceed with caution

Switch the on-screen message based on a user's role.

① Assign role = "member".

② Print "You can access the admin panel" when role == "admin", "You can access your dashboard" when role == "member", "Please log in" when role == "guest", otherwise "Unknown role".

elif is evaluated top-to-bottom

If you write if age >= 0: ... elif age >= 20: ..., the broader condition comes first, so the later one never matches. With multi-way branches, list the narrower (stricter) conditions first.

Using a value itself as the condition (truthiness)

An if condition isn't limited to comparison expressions like == — you can place a value itself there. Python will convert it to bool behind the scenes, treating "empty" or 0 as False and everything else as True.

This behavior lets you write checks like "is the input empty?" or "is anything in the cart?" very concisely.

Instead of if len(cart) > 0:, just write if cart: — the meaning is the same. This is a very Pythonic style.

# Don't run a search when the keyword is empty
search_query = ""
if search_query:
    print(f"Searching: {search_query}")
else:
    print("Please enter a keyword")
# Output: Please enter a keyword

# Move to checkout when the cart has at least one item
cart = ["apple", "bread"]
if cart:
    print(f"Proceeding with {len(cart)} item(s)")
# Output: Proceeding with 2 item(s)

Check whether a shipping address has been registered.

① Set shipping_address = "" (an empty string).

② Use if shipping_address: — print "Ready to ship" when there's an address, otherwise print "Please register your address".

③ Reassign shipping_address to something like "100 Main St, Springfield…" and run the same if again to confirm the result changes.

Writing one-line ifs with the ternary operator

When all you need is to assign one of two values based on a condition, you don't need 4 lines of if / else — you can do it in one line. This is called the ternary operator (or conditional expression). The form is value_if_true if condition else value_if_false.

# 4 lines with regular if / else
age = 25
if age >= 20:
    status = "adult"
else:
    status = "minor"

# One line with the ternary operator
status = "adult" if age >= 20 else "minor"
print(status)   # adult

The trick is using it only for simple two-way choices. When the conditions or values get long, or you have three or more branches, plain if / elif / else is more readable.

Switch the shipping label based on the order total.

① Define total_price = 3200.

② Using the ternary operator, assign "Free shipping" to shipping_label when total_price >= 5000, otherwise "Shipping $5".

③ Print shipping_label.

Range checks can be chained directly

Range checks like "0 to 100 inclusive" can be written by chaining comparison operators instead of using and. The result reads just like a math inequality, making the intent very clear.

score = 78

# Using and
if score >= 0 and score <= 100:
    print("Valid score")

# Chained form (same meaning)
if 0 <= score <= 100:
    print("Valid score")

Decide whether a test score is within the valid range (0–100).

① Define score = 78.

② Using the chained comparison 0 <= score <= 100, print "Valid score" when in range, otherwise "Out of range".

③ Reassign score = 120 and run the same check again.

Inspecting type and value (isinstance and is None)

When you want to check the type of a value held by a variable, use isinstance(value, type). Pass the value first and the type second — if the value is an instance of that type, it returns True.

price = 980
user_name = "alice"

print(isinstance(price, int))      # True
print(isinstance(user_name, str))  # True
print(isinstance(price, str))      # False

# Combine with if to compute only when it's a number
if isinstance(price, int) and price > 0:
    tax_included = int(price * 1.1)
    print(tax_included)   # 1078

What is None?

None is Python's special value meaning "no value". It represents things like "a result that hasn't been fetched yet", "an uninitialized state", or "what a function returns when it returns nothing". It is the one and only value that means "nothing", distinct from 0 or an empty string.

# A function that returns nothing implicitly returns None
def greet():
    print("hello")

result = greet()      # "hello" is printed
print(result)         # None

# An initial value meaning "not decided yet"
selected_user = None
print(selected_user)  # None

To check whether a value is None, the Pythonic way is is None rather than ==. Because None is a single, unique object in the program, comparing whether two references point to the same object in memory with is makes the intent clearer.

# User info from an API (assume it hasn't been fetched yet)
user = None

if user is None:
    print("User info not loaded")
else:
    print(f"Welcome, {user}")
# Output: User info not loaded

# To check "not None", use `is not None`
score = 0
if score is not None:
    print(f"Score: {score}")   # Score: 0

0 and None are different things

Writing if score: will treat score == 0 as False as well. When you need to distinguish "not entered (None)" from "0 points", explicitly check with if score is None: or if score is not None:.

Check the presence and type of a user's age fetched from an API.

① Define user_age = None.

② If user_age is None, print "Age not registered"; otherwise, if isinstance(user_age, int) is True, print the value of user_age.

③ Reassign user_age = 29 and run the same check again.

Checking collection contents with in / not in

To test whether a value is contained in a list, use value in collection. The opposite — "not contained" — is value not in collection. It works for lists, tuples, sets, and strings, and for dicts it checks the keys.

allowed_roles = ["admin", "editor", "member"]
current_role = "member"

if current_role in allowed_roles:
    print("Access granted")
else:
    print("Access denied")
# Output: Access granted

# Check whether a substring appears in a string
if "error" in "connection error: timeout":
    print("Detected an error line")

# For a dict, `in` checks the keys
user = {"name": "alice", "role": "admin"}
if "email" not in user:
    print("Email not registered")

Check the publication status of a blog post.

① Define published_slugs = ["python-intro", "sql-basics", "git-tips"].

② Define target = "git-tips".

③ If target is in published_slugs, print "Published"; otherwise print "Draft".

④ Change target to "draft-post" and run the same check again.

In this article, we covered advanced branching with the basics of if / if not, multi-way logic with elif / else, truthiness that treats empty values and 0 as False, the ternary operator value_1 if condition else value_2, type and value checks with isinstance and is None, and collection checks with in / not in.

In the next article, we'll learn all() / any(), which let you check whether every element in a list satisfies a condition in a single call.