| jupytext |
|
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| kernelspec |
|
- Write Boolean expressions based on English requirements
- Explain different meta-patterns of conditional blocks (nested, chained, with alternative vs. without)
- Construct various types of conditional blocks in Python
Conditionals are an example of the fundamental computational thinking concept of control flow: setting up structures in our program to control what happens when. You can think of conditionals as "forks in the road" that control what happens depending on whether some conditions are true or false.
A set of pictures might help to give the intuition:
:class: bg-primary mb-1
:width: 1000px
:align: center
Basically anytime you find yourself at a problem step or part of your problem where you say something like "do something based on / depending on / looking at / if some condition", that is a signal that you need a conditional.
Here are some examples:
- decide what to wear: check temperature, check if i'm going to a dress code location
- shopping: check how much money i have, check quality level of the thing, what i need; decide what to buy
- decide what to eat: spiciness, taste, price
What are some other real-world examples of this sort of situation that you can think of?
Here's a rough diagram of a basic conditional block in Python:
:class: bg-primary mb-1
:width: 600px
:align: center
And here's what it looks like in code
# basic conditional block
if BooleanExpression:
# do something
# maybe also something more
else:
# do something else
# and maybe even more something else
- The if statement
- The if keyword signals that a conditional block is starting.
- The Boolean expression determine where to go in the conditional block
Truegoes to the if branch;Falsegoes to the else branch
- The statement needs to end in a colon to signal that the statement has ended. This is the same as with function definitions, and as we will see, with iteration loops also.
- The "if" (
True) branch code: what should happen if the Boolean expression evaluates toTrue?- Needs to be indented because of scope (same with functions; also will be true of loops)
- The else statement:
- Signals that an else branch will be specified next
- Just the else keyword and a colon
- The "else" (false) branch code: what should happen if the Boolean expression evaluates to
False?- Also needs to be indented
Notice the idea of branch code: it's code that "belongs" to the branch. We only run it if we go into the branch.
In Python, we control what belongs to what with indentation. In other languages, you use things like curly braces (e.g., Java, Javascript).
Note: Unlike functions, if/else blocks do not create a new scope in Python. Variables assigned inside an if block are still accessible after the block finishes. The key thing indentation controls here is which code runs in which branch.
As an example, consider the following code: how many times do you think we will print the message "hello world"? Why?
# if i have passed all the requirements for graduation, print graduate! otherwise, print need to do more
# did i accumulate at least 25 credits AND earn at least a 3.0 GPA?
n_credits = 30
gpa = 3.95
hello = "hello world!"
if n_credits >= 25 and gpa >= 3.0:
print("Go ahead")
print(hello)
else:
print("Take more classes")
print(hello)
The answer: only once! Because both print statements are indented inside a conditional branch, only one branch executes: either the if branch or the else branch, but never both.
Let's look at some examples!
The following program checks if a number is even (and prints "It's even!" if so, and "It's odd!" if not).
num = 3
if num % 2 == 0:
print("It's even!")
else:
print("It's odd!")
The following program checks if a password is correct (and prints "Come in" if so, and "Go away" if not).
user_input = "hello"
password = "bunny"
# if the user input matches the password
if user_input == password:
print("Come in")
else:
print("Go away")
The following program implements a Waiter checking your age on your ID, and then shows beer/alcohol menu or says "have some water"
age = 23
drinking_age = 21
if age >= drinking_age:
print("Here's the alcohol menu")
else:
print("Have some water")
All conditional blocks depend on well-crafted Boolean Expressions, which are expressions that evaluate to (i.e., result in, produce) a Boolean value (i.e., True or False). This is what really determines the logic of the conditional control of flow. So you need to make sure you're proficient with Booolean expressions.
We use Boolean Operators to compare TWO pieces of data. When evaluated, they yield a Boolean value (True or False).
data1 booleanOperator data2
Here are the main ones:
==equal to / the same as!=not equal to / different from>greater than>=greater than or equal to (at least)<less than<=less than or equal to (at most)
We use Logical operators to combine basic Boolean expressions into more complex ones, like "is a more than 3 and less than 5"
Here are the main ones:
and(True if all Boolean expressions are True)or(True if any Boolean expression is True)not(True if the Boolean expression is not True)
Full list of comparison and logical operators here
Let's practice! Translate these Boolean expressions from English into Python.
# is the driver's speed above the limit?
speed = 75
limit = 45
# boolean expression here
# do i have a passport?
has_passport = True # assign the value True to the passport variable
# boolean expression here
# have i passed all the requirements for graduation?
# which is operationalized as "do i have enough credits, with enough GPA?"
num_credits = 120 # threshold of 120
GPA = 1.5 # threshold of 2.0
# boolean expression here
# did i take the prereq for the class OR get permission from the instructor?
took_prereq = False
have_permission = True
# boolean expression here
# is the professor in the office and the door open more than a crack (at least 15 degrees) or there is a sign that says come on in or you have an appointment?
prof_in_office = True
door_angle = 5
sign_says = "Come in"
have_appointment = True
# boolean expression here
Try these on your own! Each one requires combining Boolean operators with logical operators (and, or, not).
You can ride the roller coaster if you are at least 48 inches tall AND at least 8 years old. Write a Boolean expression that checks both conditions.
height_inches = 50
age = 7
# boolean expression here
The restaurant is open if the hour is between 11 and 14 (lunch) OR between 17 and 21 (dinner). Write a Boolean expression that checks if the restaurant is currently open. Assume the hour is in 24-hour time (e.g., 13 = 1pm).
hour = 13
# boolean expression here
You get a discount if you are a student OR a senior (65 or older).
is_student = False
age = 70
# boolean expression here
You should bring an umbrella if it is raining OR if the chance of rain is above 50% and you don't have a rain jacket.
is_raining = False
chance_of_rain = 75
has_rain_jacket = False
# boolean expression here
You can register for the class if you have taken the prerequisite AND (you are a junior or senior, based on credits: junior is at least 60, senior is at least 90).
took_prereq = True
credits = 55
# boolean expression here
A flight is delayed if the wind speed is above 40 mph OR (visibility is below 3 miles AND it is not a clear day).
wind_speed = 25
visibility = 2
is_clear = False
# boolean expression here
Now let's practice constructing conditional blocks! Follow along with me to translate these English instructions into conditional blocks. We basically need to 1) decide what the condition is and translate it into a Boolean expression, then 2) decide what actions go in the True or False branches.
If my speed is above the limit, print stop; otherwise, let me pass.
# if my speed is above the limit, print stop; otherwise, let me pass
speed = 25
limit = 45
# conditional block below here
If i have a passport, print come on in; otherwise, print go away
# if i have a passport, print come on in; otherwise, print go away
# do i have a passport?
has_passport = False # assign the value True to the passport variable
# conditional block below here
if i have passed all the requirements for graduation, print graduate! otherwise, print need to do more
# if i have passed all the requirements for graduation, print graduate! otherwise, print need to do more
# did i accumulate at least 120 credits AND earn at least a 2.0 GPA?
# did i take the prereq for the class OR get permission from the instructor?
num_credits = 110 # threshold of 120
GPA = 1.9 # threshold of 2.0
# conditional block below here
Try these on your own! Each one needs a basic if/else block.
If the temperature is below freezing (32 degrees F), print "It's freezing!"; otherwise, print "It's not freezing."
temp = 28
# conditional block below here
If the student's score is at least 60, print "Pass"; otherwise, print "Fail".
score = 55
# conditional block below here
If the user's guess matches the secret number, print "You got it!"; otherwise, print "Try again!"
guess = 7
secret_number = 4
# conditional block below here
If the item is on sale AND you have enough money (i.e., money is at least the price), print "Buy it!"; otherwise, print "Maybe next time."
on_sale = True
price = 25
money = 20
# conditional block below here
The if / else conditional block is the most basic and easy to understand. But often your programs may require something a bit simpler, and sometimes a bit more complex.
+++ {"id": "oh3kS5FLBquN"}
The else branch is actually optional. Sometimes you just want to do something if it's true, otherwise you do nothing.
The flow looks like this:
:class: bg-primary mb-1
:width: 600px
:align: center
Some examples:
- Only stop someone if they're above the speed limit
- Tell me if someone is coming!
- Look through the bag and only pull out the red skittles
Can you think of any others?
# generic
if booleanExpression:
# do something
:id: ttMHXVzySS8m
speed = 25
limit = 30
if speed > limit:
print("Stop!")
+++ {"id": "jszuKPe9CrGP"}
Keywords/phrases that signal that this is appropriate?
- if only one "choice" (or action) is described, then probably you don't need an else, since "doing nothing" is a default action
Try these on your own! Each one only needs an if — no else needed.
If the battery level is below 20, print "Low battery warning!"
battery_level = 15
# conditional block below here
If the student is on the dean's list (GPA of 3.5 or above), print a congratulations message.
gpa = 3.8
# conditional block below here
If the password is less than 8 characters long, print "Warning: password is too short!" Hint: you can use len() to get the length of a string, e.g., len("hello") returns 5.
password = "abc"
# conditional block below here
If the order total is at least $50, print "Free gift added to your order!"
order_total = 62
# conditional block below here
If it's the weekend (Saturday or Sunday), print "No class today!"
day = "Saturday"
# conditional block below here
+++ {"id": "VZzGfpRdB6ca", "tags": []}
Sometimes you have more than two mutually exclusive choices of paths (branches). In that case you need an elif.
The difference from the basic conditional is something like this:
:class: bg-primary mb-1
:width: 600px
:align: center
Some examples:
- you have a fever if you're above 100, hypothermia if you're under 95; otherwise, you're all good!
- choosing an outfit depending on where you want to go (in the Spring in Maryland!).
- choosing a football play depending on what you think the defense is showing.
Any other examples?
The key difference between this type of conditional block and the regular "if/else" blocks is that you need more than one Boolean expression; one for each if or elif statement.
+++
Here's the generic structure:
if someCondition:
# then something
elif someOtherCondition:
# then something else
else:
# some default (this is technically optional
# but if you leave it out, you may have some unexpected edge cases you didn't account for!
gpa = 3.5
gpa_threshold = 2.0
required_courses = 8
req_threshold = 10
if gpa >= gpa_threshold and required_courses >= req_threshold:
print("graduate!")
elif gpa >= gpa_threshold and required_courses < req_threshold:
print("take more required courses")
elif gpa < gpa_threshold and required_courses >= req_threshold:
print("take an easy course!")
else:
print("talk to an advisor")
:id: BaHh7pSfCCNm
# example
temp_f = 97
if temp_f >= 100:
print("fever!")
elif temp_f < 95: # need another Boolean expression
print("hypothermia!")
else:
print("all good!")
+++ {"id": "7VgOXvQWCyJH"}
Keywords/phrases that signal that this is appropriate?
When you see more than two mutually exclusive conditions or choices
+++ {"id": "19fV3icBCCu0"}
Practice! Let's translate these English instructions into Python conditional blocks.
ticket pricing: if you're under 5 or 65 and up, price is zero; if you're theater staff, you get half price (7.50); otherwise pay normal price (15)
# ticket pricing:
# if you're under 5 or 65 and up, price is zero;
# if you're theater staff, you get half price (7.50);
# otherwise pay normal price (15)
age = 65
theater_staff = True
# chained conditional block below here
help me write the grader for late assignments: if you submit before target date, you get full credit; if you submit after the target date, but before the last day of the period, you get 85% credit - if you submit on the last day of period, you get 70% credit
# help me write the updated grader for your PCEs:
# if you submit before target date, you get full credit;
# if you submit after the target date, but before or equal 1 week threshold, you get 85% credit
# if you submit after 1 week threshold, but before or equal to 2 week threshold, you get 70% credit
# otherwise, you get no credit
submission_date = 35
target_date = 36
one_week_threshold = target_date + 7
two_week_threshold = target_date + 14
score = 1
# chained conditional block below here
Try these on your own! Each one needs an if/elif/else block.
If the score is 90 or above, print "A". If 80-89, print "B". If 70-79, print "C". If 60-69, print "D". Otherwise, print "F".
score = 85
# chained conditional block below here
If the order weighs less than 1 lb, shipping is $3. If it weighs 1-5 lbs, shipping is $7. If it weighs more than 5 lbs, shipping is $12. Print the shipping cost.
weight = 3.5
# chained conditional block below here
Using 24-hour time (0 = midnight, 13 = 1pm, etc.): if the hour is less than 12, print "Good morning!" If 12-16, print "Good afternoon!" If 17-20, print "Good evening!" Otherwise, print "Good night!"
hour = 14
# chained conditional block below here
If the temperature (Celsius) is 0 or below, print "Solid (ice)". If above 100, print "Gas (steam)". Otherwise, print "Liquid (water)".
temp_c = 105
# chained conditional block below here
If BMI is below 18.5, print "Underweight". If 18.5 to 24.9, print "Normal weight". If 25.0 to 29.9, print "Overweight". If 30.0 or above, print "Obese".
bmi = 22.5
# chained conditional block below here
As your conditional blocks get more complex (especially with chained conditionals), it gets harder to keep track of all the possible situations your code needs to handle. A decision table is a simple but powerful planning tool: you list out all the possible combinations of your conditions, and then decide what should happen in each case. Think of it as a spreadsheet for your logic.
Consider the ticket pricing example above. The English description says:
- under 5 or 65 and up → free
- theater staff → half price
- otherwise → full price
But what if someone is 3 years old and theater staff? Should they get free or half price? The English description doesn't tell us! A decision table forces you to confront these situations before you write code.
Step 1: Identify your conditions (the things you're checking). For ticket pricing, that's: (a) is the person under 5 or 65+? and (b) are they theater staff?
Step 2: List every combination of True/False for those conditions. With 2 conditions, you get 4 rows:
| under 5 or 65+ | theater staff | price |
|---|---|---|
| True | True | ??? |
| True | False | $0 |
| False | True | $7.50 |
| False | False | $15 |
Step 3: Fill in what should happen for each row. That ??? is exactly the kind of edge case that causes bugs! You need to make a decision: let's say young kids and seniors always get in free, even if they're also staff. Now the table becomes:
| under 5 or 65+ | theater staff | price |
|---|---|---|
| True | True | $0 |
| True | False | $0 |
| False | True | $7.50 |
| False | False | $15 |
Step 4: Now your conditional block almost writes itself. Looking at the table, you can see that the age condition should be checked first, since it applies regardless of staff status:
age = 3
theater_staff = True
if age < 5 or age >= 65:
price = 0
elif theater_staff:
price = 7.50
else:
price = 15
print(f"Price: ${price}")
Notice how the order of the if/elif matters in our theater example! If we checked theater_staff first, we'd accidentally charge a 3-year-old staff member $7.50. This is because chained conditional branches are mutually exclusive: Python only goes down the first branch where the condition is True: the order of your conditions thus represents an implicit logic: each elif really means "if none of the above were true AND this is true."
So, two heuristics for ordering your conditions:
Specific to general (put the most restrictive conditions first)
Example:
# BROKEN — everyone with grade >= 70 gets "C"
if grade >= 70:
print("C")
elif grade >= 80:
print("B")
elif grade >= 90:
print("A")
A student with a 95 gets "C" because 95 >= 70 is True and Python stops there.
Special/edge cases first
Related to the above, you want conditions that specific special/edge cases to go first. This is the ticket pricing thing above: if you're below/above a certain edge, you're a special case (i.e., free!!).
Another example:
# Good: handle the special case before the general rule
if denominator == 0:
print("Can't divide by zero!")
elif numerator / denominator > 1:
print("Greater than 1")
else:
print("1 or less")
If you swap these, you crash on division by zero before you ever get to check for it.
Building out a decision table can help identify these specific/special edge cases, as we saw with the ticket pricing example.
Try building a decision table for the late assignment grader exercise above. What are the conditions? How many rows do you need? Are there any edge cases where it's not obvious what should happen?
+++ {"id": "_1BiX-ZSCO_O", "tags": []}
The previous problems mostly involved checking one variable against different thresholds. These problems require you to consider two or more variables together to decide what to do. Try building a decision table first to help you think through the cases!
Give a student study advice based on their grade AND how many hours they study per week. If their grade is below 70 and they study less than 10 hours, print "You need to study more!". If their grade is below 70 but they study 10 or more hours, print "Let's review your study strategies." If their grade is 70 or above and they study less than 5 hours, print "You're doing well, but don't get complacent!" Otherwise, print "Keep up the good work!"
grade = 65
study_hours = 12
# chained conditional block below here
A parking garage charges based on the type of vehicle AND how long you park. For motorcycles: $2 if 2 hours or less, $5 if more than 2 hours. For cars: $5 if 2 hours or less, $12 if more than 2 hours. For trucks: $10 if 2 hours or less, $20 if more than 2 hours. Print the fee.
vehicle = "car"
hours_parked = 3
# chained conditional block below here
Recommend a movie based on the viewer's preferred genre AND their age. If they like "action" and are under 13, print "The Incredibles". If they like "action" and are 13 or older, print "Mad Max". If they like "comedy" and are under 13, print "Despicable Me". If they like "comedy" and are 13 or older, print "The Grand Budapest Hotel". For any other genre, print "Browse the catalog."
genre = "comedy"
age = 10
# chained conditional block below here
An online store offers shipping options based on the order total AND whether the customer is a member. If the total is at least $50 and the customer is a member, print "Free 2-day shipping". If the total is at least $50 and not a member, print "Free standard shipping". If the total is less than $50 and a member, print "$3 standard shipping". If the total is less than $50 and not a member, print "$7 standard shipping".
order_total = 35
is_member = True
# chained conditional block below here
Sometimes it only makes sense to check a condition if earlier conditions are true/false. This is like a garden of forking paths or choose your own adventure. Sometimes this to save time/operations. Other times, it may make your program more readable.
I like to think of it like a choose your own adventure maze:
:class: bg-primary mb-1
:width: 600px
:align: center
Simple example: I want to know if x is the same, less than, or greater than y. We can represent this as a chained conditional with three conditions. But we can also group the less than or greater than conditional block by itself since that only make sense in the situation where x and y are not the same (i.e., x != y).
x = 5
y = 10
if x == y:
print("x and y are equal")
else:
if x < y:
print("x is less than y")
else:
print("x is greater than y")
Another simple example: graduation requirements: if you've completed the base requirements and you have a 3.0 average, then we check: do you have sufficient electives? if yes, then great! if not, take more electives. if you don't have the core requirements done, then you need to take care of that first, we'll worry about electives later.
:id: zQ5U8v4dGHI0
n_credits = 125
credit_threshold = 120
GPA = 3.5
n_electives = 3
electives_threshold = 4
if n_credits >= credit_threshold and GPA >= 3.0:
if n_electives >= electives_threshold:
print("Ready to graduate!")
else:
print("Get more electives!")
else:
print("Finish core requirements with sufficient GPA!")
The key idea behind nested conditionals is that some questions only make sense to ask after you've answered an earlier question. Think of it like a flowchart or a phone tree or a "choose your own adventure" game: the first question determines which path you're on, and then you face different follow-up questions depending on that path. This is different from chained conditionals, where all the conditions are testing for mutually exclusive possible answers for the same question (e.g., "what range is the score in?").
A good test: if you enumerate the questions that are part of your problem, and one question would be meaningless or irrelevant without knowing the answer to a another question (i.e., is dependent on a previous question), that's a signal to use nesting.
Practice! Let's translate these English instructions into Python nested conditional blocks.
Polling booth: if you don't have an id, go away and register, then come back; if you have an id come on in! then, if you need assistance, go to the assisted booth; otherwise, go to the normal booth.
# polling booth: if your registration doesn't match this location, go away to the right place; if yes, then come on in!
# then, if you need assistance, go to the assisted booth; otherwise, go to the normal booth.
registration_here = True
need_assistance = False
# nested conditional block below here
Try these on your own! Each one involves a condition that only makes sense to check after an earlier condition.
If the user is logged in, check if they have items in their cart: if yes, print "Proceeding to checkout"; if no, print "Your cart is empty". If the user is not logged in, print "Please log in first".
logged_in = True
items_in_cart = 0
# nested conditional block below here
If the rider is tall enough (at least 48 inches), then check their age: if they are under 12, print "You need an adult with you"; otherwise, print "Enjoy the ride!" If they are not tall enough, print "Sorry, you're too short for this ride".
height = 50
age = 10
# nested conditional block below here
If the restaurant is open, check if the item is on the menu. If yes, check if the item is in stock: if yes, print "Order placed!"; if no, print "Sorry, that item is sold out". If the item is not on the menu, print "We don't serve that here". If the restaurant is closed, print "Sorry, we're closed".
is_open = True
on_menu = True
in_stock = False
# nested conditional block below here
If the email is from a known contact, print "Inbox". If the email is not from a known contact: if it contains the word "unsubscribe", print "Promotions"; otherwise, print "Unknown - review manually".
from_known_contact = False
contains_unsubscribe = True
# nested conditional block below here
A user calls tech support. First, ask: is the device turning on? If it's not turning on, check if it's plugged in: if it's not plugged in, print "Plug in the device and try again"; if it is plugged in, print "The device may be broken - schedule a repair". If the device IS turning on, then check: is the screen displaying anything? If yes, print "Try restarting the application"; if no, print "The display may need replacement".
Notice how the "is it plugged in?" question only makes sense if the device isn't turning on, and the "is the screen displaying?" question only makes sense if it IS turning on. These are different follow-up questions depending on the first answer.
device_turns_on = False
is_plugged_in = False
screen_displays = True
# nested conditional block below here
First check: does the student have financial need (family income below $50,000)? If yes, check their GPA: if GPA is 3.5 or above, print "Eligible for full scholarship"; if GPA is below 3.5, print "Eligible for need-based grant". If the student does NOT have financial need, we do a different check for their GPA: if GPA is 3.8 or above, print "Eligible for merit scholarship"; otherwise, print "No financial aid available".
Notice: the GPA thresholds are different depending on whether the student has financial need. This is why nesting makes sense here -- it's not just "check income, then check GPA" independently. The meaning of the GPA check changes based on the income answer.
family_income = 45000
gpa = 3.6
# nested conditional block below here
First, check if the package is domestic or international. If domestic, check the size: if it's "small", print "Send via regular mail"; if it's "large", print "Send via ground shipping". If international, check if the destination country requires customs forms: if yes, print "Fill out customs form, then send via international courier"; if no, print "Send via international courier".
Again notice: the follow-up question for domestic packages (what size?) is completely different from the follow-up question for international packages (customs forms needed?). These are different paths with different decision points.
is_domestic = False
package_size = "small"
needs_customs = True
# nested conditional block below here
First, check if the customer has a valid receipt. If they do, check what they want: if they want a "refund", print "Process refund to original payment method"; if they want an "exchange", print "Help customer find replacement item". If they do NOT have a receipt, check if the purchase amount is under $20: if yes, print "Offer store credit as a courtesy"; if no, print "Sorry, we need a receipt for returns over $20".
Notice how having a receipt vs. not leads to entirely different sets of options. With a receipt, we ask "refund or exchange?" Without one, we check the dollar amount instead -- a completely different question.
has_receipt = False
wants = "refund"
purchase_amount = 15
# nested conditional block below here
e.g., forgetting the colon, or forgetting to indent
Best recommendation is to use templates for now as you set them up. And of course, a linter in your editor of choice (e.g., Ruff for VSCode)!
Most commonly, using = (this is assigning!) instead of == (the Boolean operator you actually want)!
Not covering all your bases or mapping the wrong conditions to outcomes. These are the trickiest because your code runs without errors, but it does the wrong thing!
Common examples:
- Checking conditions in the wrong order (e.g., a more general condition "catches" cases before a more specific one gets a chance - see discussion above on ordering of chained conditionals)
- Forgetting an edge case (e.g., what if two conditions are both true at the same time?)
- Using
andwhen you meanor, or vice versa
Especially for chained conditionals, build a decision table before you write your conditional block (see the decision tables section above) can be a great way to prevent these kinds of errors. Listing out every combination of conditions and deciding what should happen in each case helps you spot gaps and conflicts in your logic before they become bugs.
Example decision table for Project 1: https://docs.google.com/spreadsheets/d/1-q5XXbMDoji8AMVWxgUf5GW5CUOJfiWQTTV6u0-DwF8/edit?usp=sharing
You can also use decision tables to debug existing code: build the table from your code by tracing through each row, and compare it to what you intended. If they don't match, you've found your bug.
A very common mistake is writing multiple separate if statements when you actually need a chained conditional (if/elif/else). These look similar but behave very differently!
Consider this example: assign a letter grade based on a score.
Wrong (separate if blocks):
score = 85
if score >= 90:
grade = "A"
if score >= 80:
grade = "B"
if score >= 70:
grade = "C"
if score >= 60:
grade = "D"
else:
grade = "F"
print(grade)
This prints "D"! Why? Because each if is a separate conditional block — they all run independently. So even though score >= 90 is False, the program keeps going and checks every other if. When it gets to score >= 80, that's True, so grade becomes "B". But then it keeps checking: score >= 70 is also True, so grade gets overwritten to "C". Then score >= 60 is also True, so grade gets overwritten again to "D".
Right (chained conditional with elif):
score = 85
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 70:
grade = "C"
elif score >= 60:
grade = "D"
else:
grade = "F"
print(grade)
This correctly prints "B". With elif, once a condition is True and its branch runs, the rest of the chain is skipped. That's the whole point of chaining: the conditions are mutually exclusive — only one branch ever executes.
Remember that chained conditionals are really for situations where your conditions (questions) are meant to be mutually exclusive (only one should "win"), so we use elif to chain them together, which means the code can only go down a single branch depending on the answer to the mutually exclusive questions. Use separate if blocks only when the conditions are truly independent and you want each one checked regardless of the others.