Why in Python it is better to check None using is 🐍

In Python, you should not write obj == None, even if sometimes it works the same ⚠️

The reason is that == calls the comparison method eq, which can be overridden in the class — and then the behavior becomes unpredictable 🎲

For example:

class Weird:
    def eq(self, other):
        return True  # always says "equal"

obj = Weird()

print(obj == None)  # True
print(obj is None)  # False

Here obj == None gives a false result due to custom logic 🤔

Instead:

obj is None

is checks the identity of the object and cannot be overridden. Since None is a singleton, such a check is always correct and predictable ✅

Conclusion: to check for None always use is None — it is the right and safe approach 🛡️

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Do not violate the Single Responsibility Principle 🎯

A function should do one thing, and do it well.

This function does too much:

def calculate_final_total(
    price: float,
    quantity: int,
    discount_rate: float,
    tax_rate: float
) -> float:
    # Calculate the subtotal
    subtotal = price * quantity

    # Apply the discount
    discounted_amount = subtotal * (1 - discount_rate)

    # Calculate the tax
    final_total = discounted_amount * (1 + tax_rate)

    return final_total

The problem here is that the calculation of the subtotal, discount, and tax are all combined into one function. Any change to one of these steps can affect the entire calculation.

It's better to break down the logic into smaller, more specialized functions:

def calculate_subtotal(price: float, quantity: int) -> float:
    return price * quantity

def apply_discount(subtotal: float, discount: float) -> float:
    return subtotal * (1 - discount)

def calculate_tax(amount: float, tax_rate: float) -> float:
    return amount * (1 + tax_rate)

This is much better. ✅

Smaller functions with a single task are easier to test with unit tests because they have fewer dependencies and require less mocking.

Furthermore, isolated components are easier to reuse in different parts of the application or pipeline without bringing in unnecessary dependencies.

Therefore, keep your functions simple and focused.

One function – one responsibility. 📝

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📌 Marking Deprecated Code in Python: deprecated()

In large projects, old functions are not always immediately removed. They are often left for compatibility, but it's important to warn developers that they should no longer be used.

Previously,

warnings.warn()

was often used for this purpose:

import warnings

def old_api():
    warnings.warn(
        "Use new_api()",
        DeprecationWarning
    )

In Python 3.13, a

deprecated()

decorator has been introduced:

from warnings import deprecated

@deprecated("Use new_api()")
def old_api():
    return "old"

Now, the information about deprecation is part of the API itself. It can be recognized not only during runtime, but also by editors and static analysis tools.

This also works for classes:

@deprecated("Use NewClient")
class OldClient:
    pass

This is convenient for libraries, SDKs, and large projects where the API is gradually changing.

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