Fluent Python, Second Edition ● Covers 3.10, including pattern matching ● 100+ pages about type hints, with many examples ● New coverage of async/await, with examples in asyncio, FastAPI and Curio ● OReilly.com: https://bit.ly/FluPy2e Released May 2022 3
7 >>> i = 10 ** 23 >>> i 100000000000000000000000 >>> f = float(i) >>> f 1e+23 >>> i == f False >>> from decimal import Decimal >>> Decimal(i) Decimal('100000000000000000000000') >>> Decimal(f) Decimal('99999999999999991611392') >>> i | 2 100000000000000000000002 >>> f | 2 Traceback (most recent call last): File "", line 1, in TypeError: unsupported operand type(s) for |: 'float' and 'int'
8 >>> i = 10 ** 23 >>> i 100000000000000000000000 >>> f = float(i) >>> f 1e+23 >>> i == f False >>> from decimal import Decimal >>> Decimal(i) Decimal('100000000000000000000000') >>> Decimal(f) Decimal('99999999999999991611392') >>> i | 2 100000000000000000000002 >>> f | 2 Traceback (most recent call last): File "", line 1, in TypeError: unsupported operand type(s) for |: 'float' and 'int'
The “set of values” definition is not useful: Python does not provide practical ways to specify types as sets of values, except for Enum. We have very small sets (None, bool…) or extremely large ones (int, str…). 9
In practice, it’s more useful to think that int is a subtype of float because it implements the same interface, in addition to extra methods* —not because int is a subset of float** 11 *not completely true, but a useful approximation ** which it definitely isn’t
12 >>> i = 10 ** 23 >>> i 100000000000000000000000 >>> f = float(i) >>> f 1e+23 >>> i == f False >>> from decimal import Decimal >>> Decimal(i) Decimal('100000000000000000000000') >>> Decimal(f) Decimal('99999999999999991611392') >>> i | 2 100000000000000000000002 >>> f | 2 Traceback (most recent call last): File "", line 1, in TypeError: unsupported operand type(s) for |: 'float' and 'int'
Every Python value is of type object. Every Python value is of type Any. The object type implements a narrow interface, but Any is assumed to implement the widest possible interface: all possible methods! 13
Preserve the flexibility of duck typing Let your clients know what is the minimal interface expected, regardless of class hierarchies Benefits of using typing.Protocol 26 Support static analysis IDEs and linters can verify that an actual argument satisfies the protocol in the formal parameter Reduce coupling Client classes don’t need to subclass anything; just implement the protocol. This also makes testing easier.
Preserve the flexibility of duck typing Let your clients know what is the minimal interface expected, regardless of class hierarchies Benefits of using typing.Protocol 27 Support static analysis IDEs and linters can verify that an actual argument satisfies the protocol in the formal parameter Reduce coupling Client classes don’t need to subclass anything; just implement the protocol. This also makes testing easier.
Preserve the flexibility of duck typing Let your clients know what is the minimal interface expected, regardless of class hierarchies Benefits of using typing.Protocol 28 Support static analysis IDEs and linters can verify that an actual argument satisfies the protocol in the formal parameter Reduce coupling Client classes don’t need to subclass anything; just implement the protocol. This also makes testing easier.
59 59 26 Lines of code to implement all the documented functionality, with 2 constants, no imports 29 Lines of code for type hints: 7 imports, 4 definitions, and 6 overloaded signatures
Support duck typing with type hints The essence of Python’s Data Model and standard library Use typing.Protocol to build Pythonic APIs 65 Follow the Interface Segregation Principle Client code should not be forced to depend on methods it does not use Prefer narrow protocols Single method protocols should be the most common. Sometimes, two methods. Rarely more.
Support duck typing with type hints The essence of Python’s Data Model and standard library Use typing.Protocol to build Pythonic APIs 66 Follow the Interface Segregation Principle Client code should not be forced to depend on methods it does not use Prefer narrow protocols Single method protocols should be the most common. Sometimes, two methods. Rarely more.
Support duck typing with type hints The essence of Python’s Data Model and standard library Use typing.Protocol to build Pythonic APIs 67 Follow the Interface Segregation Principle Client code should not be forced to depend on methods it does not use Prefer narrow protocols Single method protocols should be the most common. Sometimes, two methods. Rarely more.
Optional at All Levels Type hints may be omitted and/or type checking disabled on single lines, functions and entire packages. Python Adopted a Gradual Type System 72 The Default type is Any Any is consistent with all other types. It is more general than object, and is understood to implement all interfaces. Does Not Catch Errors at Runtime Type hints are not used at runtime, only by code analysis tools such as linters, IDEs, and dedicated type checkers. Does Not Increase Performance Current Python implementations do not use type hints to optimize bytecode or machine code generation.
73 TypeScript by Microsoft The most successful JavaScript dialect to date. Dart by Google The language of the Flutter SDK. Hack by Facebook PHP dialect supported by the JIT-enabled HHVM runtime. Other Languages with Gradual Typing
How type hints are used 75 Run time Import time CI pipeline Linting Writing code IDE features ● Autocomplete ● Instant warnings (wiggly underlines) ● Refactoring assistance Stand-alone type checkers (same tools used for linting) Explicit type checking and services ● explicit checks with isinstance() or issubclass() ● Single dispatch functions ● Data validation (e.g. pydantic) ● API generation (e.g. FastAPI) Stand-alone type checkers ● Mypy ● Pytype ● Pyre ● Pyright CLI Data class builders ● @dataclasses.dataclass ● typing.NamedTuple metaclass
Companies with very large Python code bases invested heavily in type hints, and report they were also useful to migrate from Python 2.7. 76 Dropbox Large scale users... Employer of typing PEP authors. Released: Mypy. Facebook Employer of typing PEP authors. Released Pyre. Google Released Pytype. Microsoft Employer of typing PEP authors. Released Pyright. JetBrains Created proprietary type checker embedded in PyCharm IDE. ...and IDE vendors
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