I want to mimic this behaviour of Python3.8 in Python3.7
Positional-only parameters / was the syntax introduced to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments.
#Python3.8
def f(a,b,/,**kwargs):
print(a,b,kwargs)
>>> f(1,2,**{'a':100,'b':200,'c':300})
# 1 2 {'a': 100, 'b': 200, 'c': 300}
a,b are used only as positional parameters.
How I do the same in Python3.7
#Python3.7
def f(a,b,**kwargs):
print(a,b,kwargs)
>>> f(1,2,**{'a':1,'b':2})
# TypeError: f() got multiple values for argument 'a'
How do I make a,b to be only positional parameters. / doesn't work below from Python3.8
Is it possible to mimic / syntax in Python3.7?
You can create a custom decorator that declares the positional-only arguments, returning a wrapper that parses its own *args, **kwargs such that they fit the signature of the decorated function. Due to possible name clashes between positional-only and keyword arguments, it is not possible to use keyword-argument-packing (**) for this approach (this is the only limitation). Packed keyword arguments need to be declared either as the last positional-or-keyword parameter or as the first keyword-only parameter. Here are two examples:
def foo(a, b, kwargs): # last positional-or-keyword parameter
pass
def foo(a, *args, kwargs): # first keyword-only parameter
pass
The variable kwargs will receive the remaining **kwargs from the wrapper function, i.e. it can be used similarly as if **kwargs had been used in the decorated function directly (like in Python 3.8+).
The following implementation of the decorator is largely based on the implementation of inspect.Signature.bind with a few minor tweaks to handle positional-only parameters via the decorator-declared names and to handle the additional (artificial) kwargs parameter.
import functools
import inspect
import itertools
def positional_only(*names, kwargs_name='kwargs'):
def decorator(func):
signature = inspect.signature(func)
@functools.wraps(func)
def wrapper(*args, **kwargs):
new_args = []
new_kwargs = {}
parameters = iter(signature.parameters.values())
parameters_ex = ()
arg_vals = iter(args)
while True:
try:
arg_val = next(arg_vals)
except StopIteration:
try:
param = next(parameters)
except StopIteration:
break
else:
if param.name == kwargs_name or param.kind == inspect.Parameter.VAR_POSITIONAL:
break
elif param.name in kwargs:
if param.name in names:
msg = '{arg!r} parameter is positional only, but was passed as a keyword'
msg = msg.format(arg=param.name)
raise TypeError(msg) from None
parameters_ex = (param,)
break
elif param.default is not inspect.Parameter.empty:
parameters_ex = (param,)
break
else:
msg = 'missing a required argument: {arg!r}'
msg = msg.format(arg=param.name)
raise TypeError(msg) from None
else:
try:
param = next(parameters)
except StopIteration:
raise TypeError('too many positional arguments') from None
else:
if param.name == kwargs_name or param.kind == inspect.Parameter.KEYWORD_ONLY:
raise TypeError('too many positional arguments') from None
if param.kind == inspect.Parameter.VAR_POSITIONAL:
new_args.append(arg_val)
new_args.extend(arg_vals)
break
if param.name in kwargs and param.name not in names:
raise TypeError(
'multiple values for argument {arg!r}'.format(
arg=param.name)) from None
new_args.append(arg_val)
for param in itertools.chain(parameters_ex, parameters):
if param.name == kwargs_name or param.kind == inspect.Parameter.VAR_POSITIONAL:
continue
try:
arg_val = kwargs.pop(param.name)
except KeyError:
if (param.kind != inspect.Parameter.VAR_POSITIONAL
and param.default is inspect.Parameter.empty):
raise TypeError(
'missing a required argument: {arg!r}'.format(
arg=param.name)) from None
else:
if param.name in names:
raise TypeError(
'{arg!r} parameter is positional only, '
'but was passed as a keyword'.format(arg=param.name))
new_kwargs[param.name] = arg_val
new_kwargs.update(kwargs=kwargs)
return func(*new_args, **new_kwargs)
return wrapper
return decorator
Here is an example of how it can be used:
@positional_only('a')
def foo(a, *args, kwargs, b=9, c):
print(a, args, b, c, kwargs)
foo(1, **dict(a=2), c=3) # ok
foo(1, 2, 3, 4, 5, c=6) # ok
foo(1, b=2, **dict(a=3), c=4) # ok
foo(a=1, c=2) # error
foo(c=1) # error