Bài trước: Python căn bản (8): Python Strings (cont.)
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9. Python Booleans
9.1 Booleans
Booleans represent one of two values: True or False.
In programming you often need to know if an expression is True or False.
You can evaluate any expression in Python, and get one of two answers, True or False.
When you compare two values, the expression is evaluated and Python returns the Boolean answer
Example:
print(10 > 9)
print(10 == 9)
print(10 < 9)
When you run a condition in an if statement, Python returns True or False.
Example
Print a message based on whether the condition is True or False:
a = 200
b = 33
if b > a:
print("b is greater than a")
else:
print("b is not greater
than a")
Evaluate Values and Variables
The bool() function allows you to evaluate any value, and give you True or False in return.
Example
Evaluate a string and a number:
print(bool("Hello"))
print(bool(15))
Example
Evaluate two variables:
x = "Hello"
y = 15
print(bool(x))
print(bool(y))
Most Values are True
Almost any value is evaluated to True if it has some sort of content.
Any string is True, except empty strings.
Any number is True, except 0.
Any list, tuple, set, and dictionary are True, except empty ones.
Example
The following will return True:
print(bool("abc"))
print(bool(123))
print(bool(["apple", "cherry", "banana"]))
Some Values are False
In fact, there are not many values that evaluate to False, except empty values, such as (), [], {}, "", the number 0, and the value None. And of course the value False evaluates to False.
Example
The following will return False:
print(bool(False))
print(bool(None))
print(bool(0))
print(bool(""))
print(bool(()))
print(bool([]))
print(bool({}))
One more value, or object in this case, evaluates to False, and that is if you have an object that is made from a class with a __len__ function that returns 0 or False:
Example
class myclass():
def __len__(self):
return 0
myobj = myclass()
print(bool(myobj))
Functions can Return a Boolean
You can create functions that return a Boolean Value.
Example
Print the answer of a function:
def myFunction() :
return True
print(myFunction())
You can execute code based on the Boolean answer of a function.
Example
Print "YES!" if the function returns True, otherwise print "NO!":
def myFunction() :
return True
if myFunction():
print("YES!")
else:
print("NO!")
Python also has many built-in functions that return a boolean value, like the isinstance() function, which can be used to determine if an object is of a certain data type.
Example
Check if an object is an integer or not:
x = 200
print(isinstance(x, int))
9.2 Exercise
1. What will be the result of the following code:
print(5 > 3)?
A. True
B. False
C. 5 > 3
D. None
2. The statement below would print a Boolean value, which one?
print(bool("abc"))
A. True
B. False
C. abc
D. Null
3. The statement below would print a Boolean value, which one?
print(bool(0))
A. True
B. False
C. 0
D. Null
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The answer hints:
1(A), 2(A), 3(B)
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Cập nhật: 17/10/2024
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