[PYTHON] I read PEP 614 (Relaxing Grammar Restrictions On Decorators)

The other day, PEP 614 (Relaxing Grammar Restrictions On Decorators) became Final Commit I saw / python / peps / pull / 1437). So, this time I will read PEP 614.

Overview

• Previously, only names with ., called dotted_name, were available for decorators.
• Therefore, "expressions" such as array access and dictionary access could not be used for decorators.

  buttons = [QPushButton(f'Button {i}') for i in range(10)]

  @buttons[0].clicked.connect  # => NG
  def spam():
      ...

• Extend the grammar to accept "expressions" in decorators
• Available from Python 3.9.

approach

The grammar so far

decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE

It was

decorator: '@' namedexpr_test NEWLINE

Changed to. that's all.

It's hard to understand that. namedexpr_test is a grammatical element that points to a Python expression. The following are the expressions (from 6. Expressions — Python 3.8.3 Documentation):

• 6.1. Arithmetic conversion
• 6.2. Atom, atomic element (atom)
• 6.2.1. Identifier (or name)
• 6.2.2. Literal
• 6.2.3. Parenthesized form
• 6.2.4. Display of lists, sets and dictionaries
• 6.2.5. List display
• 6.2.6. Collective display
• 6.2.7. Dictionary display
• 6.2.8. Generator type
• 6.2.9. Yield expression
• 6.2.9.1. Generator-Iterator method
• 6.2.9.2. Usage example
• 6.2.9.3. Asynchronous generator function
• 6.2.9.4. Asynchronous generator iterator method
• 6.3. Primary
• 6.3.1. See attributes
• 6.3.2. Subscription
• 6.3.3. Slicing
• 6.3.4. Call
• 6.4. Await expression
• 6.5. Power operator
• 6.6. Unary arithmetic and bitwise operation
• 6.7. Binary arithmetic operation
• 6.8. Shifting operation
• 6.9. Binary bitwise operation
• 6.10. Comparison
• 6.10.1. Value comparison
• 6.10.2. Affiliation inspection calculation
• 6.10.3. Comparison of identity
• 6.11. Boolean operation
• 6.12. Substitution formula
• 6.13. Conditional Expressions
• 6.14. Lambda
• 6.15. List of expressions

There are various things. These are now available for decorators. Please note that some definitions may not be available (such as await).

Example

Try using a lambda expression as a decorator ...

>>> @lambda f: f
... def foo(): pass
...

Try using the ternary operator if else ...

>>> x = lambda f: f
>>> y = lambda f: f
>>> @x if True else y
... def foo(): pass
...

Of course, you can also use arrays.

>>> deco = [lambda f: f]
>>> @deco[0]
... def foo(): pass
...

Impressions

• I haven't had any problems with dotted_name so far, so I'm not so happy.
• Maybe there's something nice about the decorator array or dictionary ...?
• You need to be careful about the capacity usage because you can write very unpleasant code.

  >>> @x := staticmethod  #Substitution formula(3.From 9)
  ... def foo(): pass
  ...