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...definition is an executable statement that may contain uses and definitions of names. These references follow the normal rules for name resolution. The namespace of the class definition becomes the attribute dictionary of the class. The following operations are name binding operations. If they occur within a block, they introduce new local names in the current block unless there is also a global declaration. Function definition: def name ... Argument declaration: def f(...name...), lambda .....
...lambda x: x + i Proposal This PEP proposes the addition of a new @in clause that is a variant of the existing class and function decorator syntax. The new @in clause precedes the decorator lines, and allows forward references to the trailing function or class definition. The trailing function or class definition is always named - the name of the trailing definition is then used to make the forward reference from the @in clause. The @in clause is allowed to contain any simple statement (includ...
...lambdas are not used when they would be appropriate, just because the keyword "lambda" makes code look complex. Omitting lambda in some special cases is possible, with small and backwards compatible changes to the grammar, and provides a cheap cure against such "lambdaphobia". Rationale Sometimes people do not use lambdas because they fear to introduce a term with a theory behind it. This proposal makes introducing argumentless lambdas easier, by omitting the "lambda" keyword. itself. Imple...
...Lambda lambda's syntax does not support annotations. The syntax of lambda could be changed to support annotations, by requiring parentheses around the parameter list. However it was decided [12] not to make this change because: It would be an incompatible change. Lambdas are neutered anyway. The lambda can always be changed to a function. Accessing Function Annotations Once compiled, a function's annotations are available via the function's __annotations__ attribute. This attribute is a ...
...lambda expressions, then they might choose to write it like this: sorted_list = sorted(original, key=(lambda v: v.attr1, v.attr2)) That gets the job done, but it hardly reaches the standard of executable pseudocode that fits Python's reputation. If they don't like lambda specifically, the operator module offers an alternative that still allows the key function to be defined inline: sorted_list = sorted(original, key=operator.attrgetter(v. 'attr1', 'attr2')) Again, it get...
...lambdas Python-style instead of Lua-style x = xpcall(lambda: expression(), lambda e: default(e)) This actually returns (true, expression()) or (false, default(e)), but I think we can ignore that part. Haskell is actually similar to Lua here (except that it's all done with monads, of course): x = do catch(lambda: expression(), lambda e: default(e)) You can write a pattern matching expression within the function to decide what to do with it; catching and re-raising exceptions you don't want is...
...lambda x: x, lambda x: x**2 if f(1) == 1] (I.e. a list comprehension where the sequence following 'in' is an unparenthesized series of lambdas -- or just one lambda, even.) In Python 3.0, the series of lambdas will have to be parenthesized, e.g.: [f for f in (lambda x: x, lambda x: x**2) if f(1) == 1] This is because lambda binds less tight than the if-else expression, but in this context, the lambda could already be followed by an 'if' keyword that binds less tightly still (for details, cons...
...lambda: reduce(lambda s, a: s + a.myattr, data, 0) reduce(lambda s, a: s + a[3], data, 0) These simplify to: sum(a.myattr for a in data) sum(a[3] for a in data) List comprehensions greatly reduced the need for filter() and map(). Likewise, generator expressions are expected to minimize the need for itertools.ifilter() and itertools.imap(). In contrast, the utility of other itertools will be enhanced by generator expressions: dotproduct = sum(x*y for x,y in itertools.izip(x_vector, y_vector...
...Lambda Syntax To the best of our knowledge there is no active discussion of arrow-style lambda syntax that we are aware of, but it is nonetheless worth considering what possibilities would be ruled out by adopting this proposal. It would be incompatible with this proposal to adopt the same a parenthesized ->-based arrow syntax for lambdas, e.g. (x, y) -> x + y for lambda x, y: x + y. Our view is that if we want arrow syntax for lambdas in the future, it would be a better choice to use =&g...
...definition time. Instead, they are preserved in __annotations__ in string form. This change is being introduced gradually, starting with a __future__ import in Python 3.7. Rationale and Goals PEP 3107 added support for arbitrary annotations on parts of a function definition. Just like default values, annotations are evaluated at function definition time. This creates a number of issues for the type hinting use case: forward references: when a type hint contains names that have not been defi...
...lambda: cls_target += 1 # Error due to class scope def missing_target(): incr_x = lambda: x += 1 # Error due to missing target "x" def late_target(): incr_x = lambda: x += 1 # Error due to "x" being declared after use x = 1 lambda arg: arg += 1 # Error due to attempt to target formal parameter [x += 1 for x in data] # Error due to attempt to target iteration variable As augmented assignments currently can't appear inside scoped expressions, the above compile time name resolution...
...lambda will not be renamed. At one point lambda was slated for removal in Python 3000. Unfortunately no one was able to come up with a better way of providing anonymous functions. And so lambda is here to stay. But it is here to stay as-is. Adding support for statements is a non-starter. It would require allowing multi-line lambda expressions which would mean a multi-line expression could suddenly exist. That would allow for multi-line arguments to function calls, for instance. That is jus...
...lambda-ized annotations for methods to be able to refer to class-level definitions, by maintaining a reference to the class-level scope. This idea, too, was quickly rejected. PEP 563 summarizes the above discussion What's puzzling is PEP 563's own changes to the scoping rules of annotations—it also doesn't permit annotations to reference class-level definitions. It's not immediately clear why an inability to reference class-level definitions was enough to reject using "implicit lambda expressi...
...lambda arg: "iterable") <function <lambda> at 0x108b49110> >>> g.register(Container, lambda arg: "container") <function <lambda> at 0x108b491c8> >>> g(P()) Traceback (most recent call last): ... RuntimeError: Ambiguous dispatch: <class 'collections.abc.Container'> or <class 'collections.abc.Iterable'> Note that this exception would not be raised if one or more ABCs had been provided explicitly as base classes during class definition. In this...
...lambda functions. Example: (lambda: x := 1) # INVALID lambda: (x := 1) # Valid, but unlikely to be useful (x := lambda: 1) # Valid lambda line: (m := re.match(pattern, line)) and m.group(1) # Valid This allows lambda to always bind less tightly than :=; having a name binding at the top level inside a lambda function is unlikely to be of value, as there is no way to make use of it. In cases where the name will be used more than once, the expression is likely to need parenthesizing anyway, so th...
...lambda definition. See python-dev discussion on 17 June 2005 [2]. Motivation To allow a programming style for selectable actions that is more in accordance with the minimalistic functional programming goals of the Python language. Rationale Allow the use of None in method tables as a universal no effect rather than either (1) checking a method table entry against None before calling, or (2) writing a local no effect method with arguments similar to other functions in the table. The semantics...
...lambdas because of the single expression limitation, they must also be supported. This is done by having the expected sequence argument bound to a single parameter and then indexing on that parameter: lambda (x, y): x + y will be translated into: lambda x_y: x_y[0] + x_y[1] References [1]2to3 refactoring tool (http://svn.python.org/view/sandbox/trunk/2to3/) [2]2to3 fixer (http://svn.python.org/view/sandbox/trunk/2to3/fixes/fix_tuple_params.py) [3]IronPython (http://www.codepl...
...definitions are equivalent: def fooƛ(xℂ): return None and def foo${LATIN SMALL LETTER LAMBDA WITH STROKE}$(x${DOUBLE-STRUCK CAPITAL C}$): return None${ZERO WIDTH NO-BREAK SPACE}$ This is still easy to read and makes the full power of type-annotated Python available to ASCII believers. The typedef statement The mapping between types and declarators can be extended with this new statement. The syntax is as follows: typedef_stmt ::= "typedef" expr DECLARATOR where expr resolves to...
...definition header. Specifically, it is proposed that class definitions be able to provide a class initialisation hook as follows: class Example: def __autodecorate__(cls): # This is invoked after the class is created, but before any # explicit decorators are called # The usual super() mechanisms are used to correctly support # multiple inheritance. The class decorator style signature helps # ensure that invoking the parent class is as simple as possib...
...lambda: None) resume = getattr(type(mgr), "__resume__", lambda: None) ### --- END OF NEW STUFF --- value = type(mgr).__enter__(mgr) exc = True try: try: VAR = value # Only if "as VAR" is present PARTIAL-BLOCK-1 ### --- NEW STUFF --- suspend(mgr) tmp = yield foo resume(mgr) f(tmp) ### --- END OF NEW STUFF --- PARTIAL-BLOCK-2 except: exc = False if not exit(mgr, *sys.exc_info()): raise ...