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...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 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 expression yields an unnamed function that evaluates a single expression. It is often used for callback functions. In the example below (written using the Python 2.0 rules), any name used in the body of the lambda must be explicitly passed as a default argument to the lambda. from Tkinter import * root = Tk() Button(root, text="Click here", command=lambda root=root: root.test.configure(text="...")) This approach is cumbersome, particularly when there are several names used in t...
...lambda x: x + 1). In general, languages like that are strongly typed, so the compiler always knows the number of arguments expected and can do the right thing when presented with a functor and less arguments than expected. Python does not implement multi-argument functions by currying, so if you want a function with partially-applied arguments you would probably use a lambda as above, or define a named function for each instance. However, lambda syntax is not to everyone's taste, so say the leas...
...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: 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 In Python 2.4, the operator module gained two new functions, itemgetter() and attrgetter() that can replace common uses of the lambda keyword. The new functions run faster and are considered by some to improve readability. Pattern: lambda r: r[2] --> itemgetter(2) lambda r: r.myattr --> attrgetter('myattr') Typical contexts: sort(studentrecords, key=attrgetter('gpa')) # set a sort field map(attrgetter('lastname'), studentrecords) # extract a field Locating: grep lam...
...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 n: n*n < 50, range(100)) The takewhile code achieves the same result as the proposed syntax, albeit in a longer (some would say "less-elegant") fashion. Also, the takewhile version requires an extra function call (the lambda in the example above) with the associated performance penalty. A simple test shows that: for n in (n for n in range(100) if 1): pass performs about 10% better than: for n in takewhile(lambda n: 1, range(100)): pass though they achieve similar results. (The f...
...lambda: y ... >>> f() <function f.<locals>.<lambda> at 0x7f6f46faeae0> Anonymous functions (or functions that share a name) within the same scope will still share representations (aside from the object ID), but this is still a major improvement over the historical situation where everything except the object ID was identical. Possible Implementation Strategy This proposal has at least one titanic advantage over PEP 3150: implementation should be relatively straightf...
...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 string, item: string + chr(item), list, "") This version performs exactly the same set of string operations as the first one, but gets rid of the for loop overhead in favor of the faster, implied loop of the reduce() function. Sure, I replied, but it does so at the cost of a function call (the lambda function) per list item. I betcha it's slower, since function call overhead in Python is bigger than for loop overhead. (OK, so I had already done the comparisons. f2() t...
...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...
...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 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...
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...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 inside expressions Because lambdas use the ':' character, they cannot appear outside of parentheses in an expression. The colon is interpreted as the start of the format specifier, which means the start of the lambda expression is seen and is syntactically invalid. As there's no practical use for a plain lambda in an f-string expression, this is not seen as much of a limitation. If you feel you must use lambdas, they may be used inside of parentheses: >>> f'{(lambda x: x*2)(3)}...
...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...