【Complete Guide to the Python or Operator】 In-Depth Explanation from Basics to Advanced Usage

By 佐川 直弘 | Naohiro Sagawa (運営者)

※記事内に広告を含みます。Amazonアソシエイトとして適格販売により収入を得ています。

1. Basics of the Python `or` Operator

What is the `or` Operator?

The or operator is one of Python’s logical operators. It returns True if at least one of the two conditions is True. It only returns False when both conditions are False. This operator is especially useful when combining multiple conditions, enabling concise conditional branching.

Basic Usage

The or operator is used as follows. When two conditions are connected with or, the entire expression evaluates to True if at least one of the conditions is True.

a = 5
b = 10

if a > 3 or b < 5:
    print("At least one condition is true")
else:
    print("Both conditions are false")

In this example, a > 3 is True, while b < 5 is False. However, since the or operator is used, the overall evaluation is True as long as at least one condition is True.

Characteristics of the `or` Operator

The or operator has a feature called “short-circuit evaluation.” This means that if the left-side condition is True, the right-side condition is not evaluated and is skipped. This can be useful for optimizing performance by saving computational resources.

For example, in the following code, since the first condition is True, the b == 10 condition is not evaluated.

a = 5
b = 10

if a > 3 or b == 10:
    print("Evaluation stops here because a is greater than 3")

2. Using the `or` Operator in `if` Statements

Using `or` for Multiple Conditions

In Python’s if statements, the or operator is very useful for evaluating multiple conditions. For example, if you want to execute a specific action when at least one condition is met, using or makes the code more concise.

The following example determines actions based on the user’s age using the or operator.

age = 16

if age < 18 or age > 65:
    print("You are eligible for a discount")
else:
    print("You are not eligible for a discount")

In this example, users under 18 or over 65 are eligible for a discount. Using or allows us to combine two conditions simply.

Combining Multiple Conditions with `or`

You can simplify even more complex conditions using the or operator. In the following example, we use multiple variables to determine if at least one meets a specified condition.

temperature = 35
humidity = 70

if temperature > 30 or humidity > 60:
    print("Turn on the air conditioner")
else:
    print("No need for air conditioning")

In this case, if the temperature exceeds 30°C or humidity exceeds 60%, the program recommends turning on the air conditioner. The or operator helps simplify everyday decision-making processes.

3. Advanced Uses of the `or` Operator

Using the `or` Operator Outside of `if` Statements

The or operator is not limited to use within if statements. It can also be useful in various other situations, particularly when setting default values. You can use it to check whether a list, dictionary, or other object is None or empty and assign a default value if necessary.

def get_list(l=None):
    l = l or []
    return l

print(get_list())  # Output: []
print(get_list([1, 2, 3]))  # Output: [1, 2, 3]

In this example, if the list is None, an empty list is returned. The or operator allows for a concise way to handle default values without needing explicit conditionals.

Using Multiple `or` Conditions

In some cases, you might chain three or more conditions using or. In such cases, the conditions are evaluated from left to right, and the first True value encountered is returned.

result = None or "default" or "another"
print(result)  # Output: default

Here, since None is treated as False, the next value, "default", is returned. The or operator is useful for selecting the most appropriate value from multiple conditions.

4. Differences Between `and` and `or`

Fundamental Differences

Both and and or are logical operators, but they function very differently. The or operator returns True if at least one condition is True, while the and operator returns True only when both conditions are True.

Let’s look at an example:

a = True
b = False

if a and b:
    print("Both conditions are true")
else:
    print("At least one condition is false")

In this case, even though a is True, b is False, so the overall result is False, and the else block executes. Compared to the or operator, the and operator requires both conditions to be True for the overall result to be True.

Using `and` and `or` Together

When combining and and or in a single condition, it’s recommended to use parentheses to make the logic clear.

temperature = 25
weather = "rainy"

if (temperature > 20 and temperature < 30) or weather == "rainy":
    print("Take an umbrella with you")
else:
    print("No umbrella needed")

In this case, the program advises taking an umbrella if the temperature is between 20°C and 30°C or if the weather is rainy. By combining and and or, you can construct complex conditions concisely.

5. Optimizing Performance with the `or` Operator

Performance Benefits of Short-Circuit Evaluation

One of the biggest advantages of the or operator is its “short-circuit evaluation” property. If the left-hand condition evaluates to True, the right-hand condition is not evaluated, preventing unnecessary computations and improving performance.

For example, in the following code, can_publish() is never executed because can_edit() returns True:

def can_edit():
    return True

def can_publish():
    print("This function will not be called")
    return True

if can_edit() or can_publish():
    print("Editing or publishing is allowed")

Since the left-hand condition can_edit() returns True, the or operator does not evaluate the right-hand condition can_publish(). This behavior is particularly useful for improving efficiency in large-scale applications.

Practical Use Cases

When handling large datasets or processing multiple conditions efficiently, short-circuit evaluation helps reduce execution time. Using the or operator strategically allows you to avoid unnecessary operations and write optimized code.

6. Conclusion

Reviewing the `or` Operator

The or operator plays a crucial role in Python programming, particularly in logical operations. It returns True if at least one condition is True, making it especially useful in conditional statements and default value settings. Additionally, since or supports short-circuit evaluation, it contributes to performance optimization.

Try It in Your Code

Beyond just learning the theory, it’s highly recommended to practice using the or operator in a Python development environment. Try different use cases, from basic applications to advanced scenarios, and observe how the code behaves. By experimenting with conditions and writing efficient code, you’ll develop a deeper understanding of the or operator.