Why Isn't My if Statement Checking All Conditions with and/or?
You're building more sophisticated programs that need to make decisions based on multiple factors. You've mastered basic if/elif/else statements, and now you want to check if several things are true at once, or if at least one thing is true. This is where and, or, and not come in.
Let's say you're writing a program for a video game that checks if a player is eligible for a special bonus. The rules are:
The player must have more than 100 points AND have completed level 5.
You write the code, and it seems straightforward enough:
points = 120
level_completed = 4 # Should be level 5 or higher
if points > 100 and level_completed >= 5:
print("Player is eligible for bonus!")
else:
print("Player is NOT eligible for bonus.")
You run this, expecting "Player is NOT eligible for bonus." (because level_completed is 4). And indeed, that's what you get. Perfect!
But then you get confused by a slightly different scenario. You want to check if a player is in the "beginner" category, meaning:
Their score is less than 50 OR their current level is less than 3.
You try this:
score = 60
current_level = 1
# Goal: If score < 50 OR current_level < 3
if score < 50 or current_level < 3:
print("Player is a beginner.")
else:
print("Player is NOT a beginner.")
You expect "Player is a beginner" (because current_level is 1, which is less than 3). And again, it works.
So why do people get stuck? The confusion comes when they try to combine these in a way that seems intuitive but isn't how Python processes it. For example, trying to check if a number is between 10 and 20:
number = 5
# This is a common beginner mistake!
if number > 10 or < 20:
print("This will cause a SyntaxError!")
This will instantly crash your program with a SyntaxError. The problem isn't and or or themselves, but how you construct the individual conditions around them. Let's demystify these powerful operators.
The Solution: Build Each Condition Independently
The key to using and, or, and not correctly is to remember that each side of these operators must be a complete, self-contained condition that evaluates to True or False on its own.
Python doesn't "remember" the number variable when it sees < 20. It needs to be told explicitly.
1. and: Both Must Be True
The and operator returns True only if all the conditions it connects are True. If even one condition is False, the entire and expression is False.
Problematic (SyntaxError): if number > 10 and < 20:
Correct Way: You must repeat the variable for each comparison:
number = 15
# Correctly checking if number is between 10 and 20 (inclusive)
if number >= 10 and number <= 20:
print(f"{number} is between 10 and 20.") # Output: 15 is between 10 and 20.
number = 5
if number >= 10 and number <= 20:
print(f"{number} is between 10 and 20.") # No output, as 5 >= 10 is False
Pythonic Shortcut for Ranges: For checking if a number is within a range, Python offers a beautiful, more readable shortcut:
number = 15
if 10 <= number <= 20: # This is equivalent to number >= 10 and number <= 20
print(f"{number} is within the range.")
2. or: At Least One Must Be True
The or operator returns True if at least one of the conditions it connects is True. It only returns False if all conditions are False.
Problematic (SyntaxError): if day == "Saturday" or "Sunday":
Correct Way: Again, each comparison must be complete:
day = "Saturday"
# Correctly checking if day is Saturday OR Sunday
if day == "Saturday" or day == "Sunday":
print("It's the weekend!") # Output: It's the weekend!
day = "Monday"
if day == "Saturday" or day == "Sunday":
print("It's the weekend!") # No output, as both conditions are False
3. not: Reversing a Condition
The not operator is simpler: it simply reverses the boolean value of a condition. If a condition is True, not makes it False, and vice-versa.
is_admin = False
if not is_admin: # This is equivalent to saying 'if is_admin == False:'
print("Access Denied (not an admin).") # Output: Access Denied (not an admin).
has_permission = True
if not has_permission:
print("You don't have permission.") # No output, as not True is False
Operator Precedence (Like PEMDAS for Logic)
Just like math has an order of operations (PEMDAS/BODMAS), logical operators also have precedence:
not(highest)andor(lowest)
This means not is evaluated first, then and, then or. If you want to change this order, use parentheses (), just like in math.
# Example: Is it the weekend AND it's NOT raining?
day = "Saturday"
is_raining = False
if (day == "Saturday" or day == "Sunday") and not is_raining:
print("Perfect weekend weather!") # Output: Perfect weekend weather!
Here, (day == "Saturday" or day == "Sunday") is evaluated first because of the parentheses.
Frequently Asked Questions (FAQs)
1. Why does Python give a SyntaxError for if number > 10 or < 20?
A SyntaxError means Python doesn't understand the structure of your code. In or < 20, < 20 isn't a complete expression. Python expects something on both sides of the comparison operator (<).
2. Can I use many ands and ors together?
Yes, you can chain them, but use parentheses () to keep your logic clear. Code that's hard to read is often buggy.
3. What if I want to check if a variable is one of several values?
Instead of if x == 1 or x == 2 or x == 3:, a more "Pythonic" and cleaner way is to use the in operator with a list or tuple:
if x in [1, 2, 3]:
This checks if x is present in the collection [1, 2, 3].
Conclusion: Mastering Complex Decisions
The and, or, and not operators are incredibly powerful tools for controlling the flow of your Python programs. The "trick" is simply to remember that each condition you combine must be a standalone statement that can be evaluated to True or False.
You've now moved beyond simple single decisions to constructing intricate logical pathways, allowing your programs to respond intelligently to a multitude of situations. This is a huge leap towards writing truly dynamic and smart code.
What's the most complex logical condition you're excited to implement in your Python projects? Let us know!
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