This question already has answers here:
Is there a builtin identity function in python?
(10 answers)
Closed 3 years ago.
Is there any function in python standard library that returns its argument, something similar to lambda x: x and a method that returns its self?
My motivation is to implement something like:
def identity(x):
return x
def default(arg, default, func=identity):
return func(arg) if arg is not None else func(default)
Ruby has itself that would be something like:
class Self:
def itself(self):
return self
There are other use cases for a function/method composition that returns its arguments or its self.
Imho there are two packages that bring identity along many other functional, pure python, and performant methods: funcy and toolz
Related
This question already has answers here:
How to implement built-in sum() of the class?
(2 answers)
Define "sum" for a class using non-associative addition
(2 answers)
Closed 9 months ago.
Is there any way I can over write the sum() method to act differently on a list?
I expected I would be able to find a dunder method that would allow me to define my own implementation, but I couldn't.
That's how I'd imagine the implementation
class MyDummyList(list):
def __sum__(self): -> str:
return 'a'
x = MyDummyList()
print(sum(x)) -> # prints 'a'
Obviously this breaks as there is no dunder method __sum__ but I am somewhat surprised that there is no such a way to do that using dunder methods as sum is a quite common operation on iterables.
This question already has answers here:
What does -> mean in Python function definitions?
(11 answers)
Python3 function definition, arrow and colon [duplicate]
(3 answers)
What does the -> (dash-greater-than arrow symbol) mean in a Python method signature? [duplicate]
(1 answer)
Closed 9 months ago.
I'm analyzing some old code that I've inherited, and I have a question about the use of "self" and "None" keywords, specifically in the following example:
def run(self) -> None:
I understand that the self keyword is similar to the "this" keyword in C++ in that, in conjunction with the dot operator, it allows us to access the attributes and methods of the class in question. What I'm really interested in is the use of "-> None" in the declaration of the method named "run." Is this in PEP 8 because I can't find an example. I'm using Python 3.7, in case that matters.
What is the purpose of writing a method in this manner? What does "-> None" do?
They're called type hints, and they enable annotating the types of the parameters and return types of functions.
https://peps.python.org/pep-0484/
This question already has answers here:
PyCharm, what is python_stubs?
(2 answers)
Finding the source code for built-in Python functions?
(8 answers)
Closed 1 year ago.
If I go to the implementation of let's say the next() builtin function, I'm forwarded to the builtins.py module to following code:
def next(iterator, default=None): # real signature unknown; restored from __doc__
"""
next(iterator[, default])
Return the next item from the iterator. If default is given and the iterator
is exhausted, it is returned instead of raising StopIteration.
"""
pass
Now, it looks like this functions does nothing but obviously that's not the case.
I understand that this function is implemented in C under the hood, but how and when is this function(or other builtin functions) mapped to the underlying C implementation?
If you have an answer to this question, can you please also provide links that I can read in order to better unterstand this topic?
I'm not asking, where the code is, but how and when the function is mapped to that code
Thank you.
This question already has answers here:
Is there a builtin identity function in python?
(10 answers)
Closed 7 years ago.
In functional programming is sometimes useful to have an identity function.
Is there a built-in or a function defined in some module that does this?
The identity function can be simply defined as:
identity = lambda x: x
I'm not aware of this function defined in any module, but it could be a good fit for functools.
This question already has answers here:
Creating functions (or lambdas) in a loop (or comprehension)
(6 answers)
Closed 6 months ago.
in order to automatically generate parameterized tests, I am trying to add methods to a class in by freezing some parameters of an existing method. Here is the piece of Python 3 code
class A:
def f(self, n):
print(n)
params = range(10)
for i in params:
name = 'f{0}'.format(i)
method = lambda self: A.f(self, i)
setattr(A, name, method)
However, the following lines then produce rather disappointing output
a = A()
a.f0()
prints "9" (instead of "0"). I must be doing something wrong, but I can't see what. Can you help ?
Thanks a lot
Edit: this question is indeed a duplicate. I would like to acknowledge the quality of all comments, which go much deeper than the raw answer.
Try
method = lambda self, i=i: A.f(self, i)
because otherwise when you call the method i's value may have changed
The best way to "freeze" parameters in Python is to use functools.partial. It's roughly equivalent to warwaruk's lambda version, but if you have a function with lots of arguments yet only want to freeze one or two of them (or if you only know certain arguments and don't care about the rest) using partial is more elegant as you only specify the arguments you want to freeze rather than having to repeat the whole function signature in the lambda.
An example for your program:
class A:
def f(self, n):
print(n)
from functools import partial
for i in range(10): # params
setattr(A, 'f{0}'.format(i), partial(A.f, n=i))
Depending on which version of Python 3 you're using, you may not need to include the 0 in the string format placeholder; starting with 3.1, iirc, it should be automatically substituted.