The CQS (Command Query Separation) Design Principle

When writing code, clarity and maintainability are everything. One of the most powerful design principles you can apply to achieve this is CQS - Command Query Separation.

What is CQS?

CQS (Command Query Separation) means:

A function should either do something (a Command) or answer something (a Query) - never both.

• Commands: Change system state, return nothing
• Queries: Return data, change nothing

By applying this rule, your code becomes easier to read, test, and maintain.

Bad Example

Here's an example that mixes a query and a command in a single function:

def configure_interface(device, interface):
    status = device.get_interface_status(interface)  # ← Query
    if status == "down":
        device.enable_interface(interface)           # ← Command
    return status                                    

Why it's bad:

• Does two things: checks state + changes state
• Harder to test
• Breaks single-responsibility principle

Good Example

Here’s an example that demonstrates clear separation using the Command Query Separation (CQS) principle:

# Query – no state change
def is_interface_down(device, interface):
    return device.get_interface_status(interface) == "down"

# Command – state change, no return
def enable_interface(device, interface):
    if not device.has_interface(interface):
        raise ValueError(f"Interface {interface} does not exist")
    device.enable_interface(interface)

Usage

if is_interface_down(device, "Gig0/1"):    # ← Query
    enable_interface(device, "Gig0/1")     # ← Command

Final Thoughts

Applying CQS in your code helps you:

• Avoid hidden side effects
• Improve testability
• Write clearer, more predictable logic

Thanks for reading — and happy automating!