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Testing & modern typing

pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.

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Format Data with Type Hints in Python

Build a validated person dict with modern type hints and optional list handling.

from typing import Any, Dict, List, Optional, Union

JsonValue = Union[str, int, float, bool, None, List["JsonValue"], Dict[str, "JsonValue"]]

def format_person(name: str, age: int, hobbies: Optional[List[str]] = None) -> Dict[str, Any]:
    """Build a person dict with validated typing."""
    if not name or age < 0:…

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Testing & modern typing easy

How to Merge TypedDicts in Python

Merge two TypedDict dictionaries with type-aware logic using NotRequired, **kwargs unpacking, and safe key updates.

typing typeddict dict

Python

from typing import TypedDict, NotRequired, merge  # hypothetical

class User(TypedDict):
    name: str
    email: NotRequired[str]
    age: NotRequired[int]

def merge_users(base: User, **overrides: User) -> User:
    """Merge two user dicts with typing-aware logic."""
    result: User = dict(base)
    for key, value …

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Testing & modern typing easy

How to Validate Dataclass Fields with Python Type Hints

A beginner-friendly helper that checks if instance attributes match their declared type hints using dataclasses and get_type_hints.

dataclasses type-hints validation

Python

from typing import Any, TypeVar, get_type_hints
from dataclasses import dataclass

T = TypeVar("T")

@dataclass
class User:
    name: str
    age: int
    email: str

def validate_fields(obj: Any) -> dict[str, bool]:
    """Check if object attributes match declared type hints."""
    hints = get_type_hints(obj.__class…

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Testing & modern typing easy

How to Write a Fast Smoke Test for a Critical Path in Python

A quick smoke test that validates the /health critical path executes fast enough, raising errors on wrong paths or slow responses.

smoke-test performance health-check

Python

import time

def smoke_test(path):
    if path != "/health":
        raise ValueError("Critical path expected /health")
    start = time.perf_counter()
    # Simulate the critical health check work
    time.sleep(0.01)
    elapsed = time.perf_counter() - start
    if elapsed > 0.05:
        raise RuntimeError("Health …

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Testing & modern typing easy

How to freeze time in Python tests with freezegun

Use the freezegun decorator to freeze datetime.now() at a fixed timestamp so tests that depend on current time run deterministically.

freezegun datetime testing

Python

from datetime import datetime
from freezegun import freeze_time


@freeze_time("2024-01-15 12:30:00")
def test_frozen_time():
    now = datetime.now()
    return now


if __name__ == "__main__":
    result = test_frozen_time()
    print(result)

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Testing & modern typing easy

Mock datetime with time-machine in Python

Use the time-machine library to travel to a fixed datetime when running tests or scripts, mocking datetime.utcnow().

testing datetime mock

Python

from time_machine import travel
from datetime import datetime


@travel("2020-01-01 10:30:00")
def check_date():
    return datetime.utcnow()


if __name__ == "__main__":
    print(check_date())

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Testing & modern typing easy

Mock datetime.now to freeze time in Python

Use unittest.mock.patch to replace datetime.now with a fixed value so your code always sees the same time during tests.

datetime mock unittest

Python

from datetime import datetime
from unittest.mock import patch

def current_message():
    now = datetime.now()
    return f"Current time: {now:%Y-%m-%d %H:%M:%S}"

if __name__ == "__main__":
    with patch("__main__.datetime") as mock_dt:
        mock_dt.now.return_value = datetime(2024, 3, 15, 10, 30, 0)
        prin…

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