Type checking using Python 3.x annotations (Python)

2013-05-23T22:46:19-07:00

Python recipe 578528 by David Mertz (annotations, check, decorator, python3, type, typecheck). Revision 5.

Some other recipes have been suggested to allow type checking by various means. Some of these require the use of type specification in a decorator itself. Others try to be much more elaborate in processing a large variety of annotations (but hence require much more and more convoluted code).

The recipe provided below is very short, and simply provides actual type checking of arguments and return values. It utilizes an unadorned decorator, rather than manufacture one that is parameterized by types or other arguments.

Dynamic function parameters using annotations (Python)

2012-02-22T22:31:02-08:00

Python recipe 578049 by pavel (annotations, decorator).

Python default values for keyword arguments are evaluated only once and not on every function call. For example following function will not work as user may expect:

def printNow(l=[], now=datetime.now()):
    l.append(len(l))

    print('List:', l, ' id:', id(l))
    print('Now:', now)


for i in range(3):
    printNow()
    print()

The "dynamic" decorator solves problem by evaluating callables, that are assigned to parameters using annotations syntax (see PEP 3107).

Auto assign self attributes in init using PEP 484 (Python)

2017-04-26T17:26:29-07:00

Python recipe 580790 by Ryan Gonzalez (annotations, class, self, type). Revision 2.

TL;DR: Short way of doing stuff like def __init__(self, a, b): self.a = a; self.b = b, using type annotations from PEP 484, inspired by Dart's MyConstructor(this.a, this.b) and Ruby's/Crystal's/(Moon|Coffee)Script's constructor/initialize(@a, @b).

Reevaluate functions when called, v3 (Python)

2009-05-14T15:18:41-07:00

Python recipe 576754 by geremy condra (annotations, decorator, function). Revision 3.

This small snippet came about as a result of this discussion on python-ideas, requesting a new syntax for dynamically reevaluating a function each time it is called.

It is a minor alteration of version 2 of this recipe that, instead of calling eval() on string annotations, simply requires that the annotations be callable and calls them at runtime.

Reevaluate functions when called, v2 (Python)

2009-05-14T15:10:51-07:00

Python recipe 576753 by geremy condra (annotations, decorator, function).

As with version 1 of this recipe, it was sparked by a discussion on python-ideas about adding a special syntax to function signatures for reevaluating the arguments to a function at runtime. The below is a decorator and annotation based solution to this problem which stores the code to be evaluated as a string in the annotations, rather than reevaluating the entire function every time it is called.

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