Hi,
For technical reasons, many functions of the Python standard libraries
implemented in C have positional-only parameters. Example:
-------
$ ./python
Python 3.7.0a0 (default, Feb 25 2017, 04:30:32)
>>> help(str.replace)
replace(self, old, new, count=-1, /) # <== notice "/" at the end
...
>>> "a".replace("x", "y") # ok
'a'
>>> "a".replace(old="x", new="y") # ERR!
TypeError: replace() takes at least 2 arguments (0 given)
-------
When converting the methods of the builtin str type to the internal
"Argument Clinic" tool (tool to generate the function signature,
function docstring and the code to parse arguments in C), I asked if
we should add support for keyword arguments in str.replace(). The
answer was quick: no! It's a deliberate design choice.
Quote of Yury Selivanov's message:
"""
I think Guido explicitly stated that he doesn't like the idea to
always allow keyword arguments for all methods. I.e. `str.find('aaa')`
just reads better than `str.find(needle='aaa')`. Essentially, the idea
is that for most of the builtins that accept one or two arguments,
positional-only parameters are better.
"""
http://bugs.python.org/issue29286#msg285578
I just noticed a module on PyPI to implement this behaviour on Python functions:
https://pypi.python.org/pypi/positional
My question is: would it make sense to implement this feature in
Python directly? If yes, what should be the syntax? Use "/" marker?
Use the @positional() decorator?
Do you see concrete cases where it's a deliberate choice to deny
passing arguments as keywords?
Don't you like writing int(x="123") instead of int("123")? :-) (I know
that Serhiy Storshake hates the name of the "x" parameter of the int
constructor ;-))
By the way, I read that "/" marker is unknown by almost all Python
developers, and [...] syntax should be preferred, but
inspect.signature() doesn't support this syntax. Maybe we should fix
signature() and use [...] format instead?
Replace "replace(self, old, new, count=-1, /)" with "replace(self,
old, new[, count=-1])" (or maybe even not document the default
value?).
Python 3.5 help (docstring) uses "S.replace(old, new[, count])".
Victor
I just spent a few minutes staring at a bug caused by a missing comma
-- I got a mysterious argument count error because instead of foo('a',
'b') I had written foo('a' 'b').
This is a fairly common mistake, and IIRC at Google we even had a lint
rule against this (there was also a Python dialect used for some
specific purpose where this was explicitly forbidden).
Now, with modern compiler technology, we can (and in fact do) evaluate
compile-time string literal concatenation with the '+' operator, so
there's really no reason to support 'a' 'b' any more. (The reason was
always rather flimsy; I copied it from C but the reason why it's
needed there doesn't really apply to Python, as it is mostly useful
inside macros.)
Would it be reasonable to start deprecating this and eventually remove
it from the language?
--
--Guido van Rossum (python.org/~guido)
> To me, 'pop' implies mutation. Tuples do not have a pop method, and it
> is not obvious to me that either tuples or frozensets should. What are > the use cases that are not better served by converting to list or set?
> --
> Terry Jan Reedy
1) coverting to set or list is O(n) in time
2) if I have to keep the old copy,
standard set solution will be O(n) both in time and space!
working examples:
1) priority queue:
insert and pop occur
2) share immutable data to difference subroutines:
each one can modify local copy safely and concurrency.
dict.get() is a very useful method, but for lists and tuples, we need to
rely on try/except instead.
Can we get list.get and tuple.get as well?
Also, for list, a list.setdefault like the dict.setdefault would be logical.
where we use types?
almost:
isinstance(obj, T);
# issubclass(S, T);
Note that TYPE is SET;
if we add __contains__ and __le__ into "type",
then things become:
obj in T;
# S <= T; # if only not misleading to a total ordering
example:
def __getitems__(self, i):
if i in Integral:
...
elif i in slice:
...
# Save "(,)". Really, I prefer to type "lambda:;" than "()".
# I fail to modify the "type" object, since it is a C-object.
Howdy All!
This suggestion is inspired by the question on "Efficient debug logging".
I propose a keyword to mark an expression for delayed/lazy execution, for
the purposes of standardizing such behavior across the language.
The proposed format is:
delayed: <expr>
i.e. log.info("info is %s", delayed: expensiveFunction())
Unlike 'lambda' which returns a function (so the receiver must be
lambda-aware), delayed execution blocks are for all purposes values. The
first time the value (rather than location) is read, or any method on the
delayed object is called, the expression is executed and the delayed
expression is replaced with the result. (Thus, the delayed expression is
only every evaluated once).
Ideally:
a = delayed: 1+2
b = a
print(a) #adds 1 and 2, prints 3
# a and b are now both just 3
print(b) #just prints 3
Mechanically, this would be similar to the following:
class Delayed():
def __init__(self, func):
self.__func = func
self.__executed = False
self.__value = None
def __str__(self):
if self.__executed:
return self.__value.__str__()
self.__value = self.__func()
self.__executed = True
return self.__value.__str__()
def function_print(value):
print('function_print')
print(value)
def function_return_stuff(value):
print('function_return_stuff')
return value
function_print(function_return_stuff('no_delay'))
function_print(Delayed(lambda: function_return_stuff('delayed')))
delayed = Delayed(lambda: function_return_stuff('delayed_object'))
function_print(delayed)
function_print(delayed)
Unfortunately, due to
https://docs.python.org/3/reference/datamodel.html#special-lookup , this
magic delayed class would need to implement many magic methods, as
__getattribute__ is not _always_ called.
I have seen some interest into lazy functionality implementation.
I wondered if it can be linked with optional import.
PEP 8 <http://www.python.org/dev/peps/pep-0008/> authoritatively states:
Imports are always put at the top of the file, just after any module
comments and docstrings, and before module globals and constants.
So, if we want to stick to PEP8 with non mandatory import, we have to catch
the import errors, or jail the class or function using extra functionnality.
Why not using the potential lazy keyword to have a nice way to deal with it?
For example:
lazy import pylab as pl # do nothing for now
>
> # do stuff
>
> def plot(*args):
> pl.figure() # Will raise an ImportError at this point
> pl.plot(...)
>
That way, our library will raise an ImportError only on plot func usage
with an explicit traceback : if matplotlib is not installed, we will have
the line where it is used for the first time and we will have the name of
the faulty library.
Hi, all
I have a suggestion that, the sort() member method of the list instance, should return the 'self' as the result of list.sort() call.
Now list.sort() returns nothing, so that I can NOT write code like this:
res = {item: func(item) for item in item_list.sort()}
It feels bad.
Regards!
Hi, everyone!
Oftenly, __repr__ return "{type}({args}, {kws})"
problem:
1) to call .format() myself is tedious
I have seen someone do it wrong: "T(1,)"!
I write a helper function myself,
but to import such tiny function everywhere isnot good.
2) pprint cannot dive into string that from repr()
To use pprint, I sometimes have to recursively
turn objects into builtin containers:
(type_name, args...)
solution:
allow __repr__ to return
str
OR tuple: (args, kws)
Better, isn't it?
We have the immutable frozenset for sets and and tuples for lists.
But we also have something to manipulate dict as immutable datastructures:
>>> from types import MappingProxyType as idict
>>> d = idict({'a':1, 'b':2, 'c':3})
>>> d['a'] = 4
Traceback (most recent call last):
File "<ipython-input-3-c6f93d6278af>", line 1, in <module>
d['a'] = 4
TypeError: 'mappingproxy' object does not support item assignment
We could expose this as a built type to allow the last of the most
important data structure in Python to be easily immutable.
