您可以列出函数接收的关键字参数吗?

问题:您可以列出函数接收的关键字参数吗?

我有一个字典,我需要将键/值作为关键字参数传递..例如。

d_args = {'kw1': 'value1', 'kw2': 'value2'}
example(**d_args)

这可以正常工作,但是如果d_args字典中有一些example函数不接受的值,则它显然会死掉。.说,如果将示例函数定义为def example(kw2):

这是一个问题,因为我无法控制d_argsexample函数的生成。它们都来自外部模块,并且example仅接受dict中的某些关键字参数。

理想情况下,我会做

parsed_kwargs = feedparser.parse(the_url)
valid_kwargs = get_valid_kwargs(parsed_kwargs, valid_for = PyRSS2Gen.RSS2)
PyRSS2Gen.RSS2(**valid_kwargs)

我可能只是从有效的关键字参数列表中过滤出dict,但是我想知道:是否有一种方法可以以编程方式列出特定函数所采用的关键字参数?

I have a dict, which I need to pass key/values as keyword arguments.. For example..

d_args = {'kw1': 'value1', 'kw2': 'value2'}
example(**d_args)

This works fine, but if there are values in the d_args dict that are not accepted by the example function, it obviously dies.. Say, if the example function is defined as def example(kw2):

This is a problem since I don’t control either the generation of the d_args, or the example function.. They both come from external modules, and example only accepts some of the keyword-arguments from the dict..

Ideally I would just do

parsed_kwargs = feedparser.parse(the_url)
valid_kwargs = get_valid_kwargs(parsed_kwargs, valid_for = PyRSS2Gen.RSS2)
PyRSS2Gen.RSS2(**valid_kwargs)

I will probably just filter the dict, from a list of valid keyword-arguments, but I was wondering: Is there a way to programatically list the keyword arguments the a specific function takes?


回答 0

比起直接检查代码对象并计算出变量,更好的方法是使用检查模块。

>>> import inspect
>>> def func(a,b,c=42, *args, **kwargs): pass
>>> inspect.getargspec(func)
(['a', 'b', 'c'], 'args', 'kwargs', (42,))

如果您想知道它是否可以与一组特定的args一起调用,则需要未指定默认值的args。这些可以通过以下方式获得:

def getRequiredArgs(func):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if defaults:
        args = args[:-len(defaults)]
    return args   # *args and **kwargs are not required, so ignore them.

然后一个函数来告诉您特定字典缺少什么:

def missingArgs(func, argdict):
    return set(getRequiredArgs(func)).difference(argdict)

同样,要检查无效的参数,请使用:

def invalidArgs(func, argdict):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if varkw: return set()  # All accepted
    return set(argdict) - set(args)

因此,是否可以调用的完整测试是:

def isCallableWithArgs(func, argdict):
    return not missingArgs(func, argdict) and not invalidArgs(func, argdict)

(这仅在python的arg解析方面是好的。任何运行时检查kwargs中的无效值显然都无法检测到。)

A little nicer than inspecting the code object directly and working out the variables is to use the inspect module.

>>> import inspect
>>> def func(a,b,c=42, *args, **kwargs): pass
>>> inspect.getargspec(func)
(['a', 'b', 'c'], 'args', 'kwargs', (42,))

If you want to know if its callable with a particular set of args, you need the args without a default already specified. These can be got by:

def getRequiredArgs(func):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if defaults:
        args = args[:-len(defaults)]
    return args   # *args and **kwargs are not required, so ignore them.

Then a function to tell what you are missing from your particular dict is:

def missingArgs(func, argdict):
    return set(getRequiredArgs(func)).difference(argdict)

Similarly, to check for invalid args, use:

def invalidArgs(func, argdict):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if varkw: return set()  # All accepted
    return set(argdict) - set(args)

And so a full test if it is callable is :

def isCallableWithArgs(func, argdict):
    return not missingArgs(func, argdict) and not invalidArgs(func, argdict)

(This is good only as far as python’s arg parsing. Any runtime checks for invalid values in kwargs obviously can’t be detected.)


回答 1

这将打印所有可传递参数,关键字和非关键字参数的名称:

def func(one, two="value"):
    y = one, two
    return y
print func.func_code.co_varnames[:func.func_code.co_argcount]

这是因为第一个co_varnames始终是参数(下一个是局部变量,y如上例所示)。

所以现在您可以拥有一个功能:

def getValidArgs(func, argsDict):
    '''Return dictionary without invalid function arguments.'''
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    return dict((key, value) for key, value in argsDict.iteritems() 
                if key in validArgs)

然后可以这样使用:

>>> func(**getValidArgs(func, args))

编辑:一个小的补充:如果您真的只需要一个函数的关键字参数,则可以使用func_defaults属性来提取它们:

def getValidKwargs(func, argsDict):
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    kwargsLen = len(func.func_defaults) # number of keyword arguments
    validKwargs = validArgs[-kwargsLen:] # because kwargs are last
    return dict((key, value) for key, value in argsDict.iteritems() 
                if key in validKwargs)

现在,您可以使用已知的args调用函数,但是要提取kwargs,例如:

func(param1, param2, **getValidKwargs(func, kwargsDict))

假定签名中func不使用no *args**kwargsmagic。

This will print names of all passable arguments, keyword and non-keyword ones:

def func(one, two="value"):
    y = one, two
    return y
print func.func_code.co_varnames[:func.func_code.co_argcount]

This is because first co_varnames are always parameters (next are local variables, like y in the example above).

So now you could have a function:

def getValidArgs(func, argsDict):
    '''Return dictionary without invalid function arguments.'''
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    return dict((key, value) for key, value in argsDict.iteritems() 
                if key in validArgs)

Which you then could use like this:

>>> func(**getValidArgs(func, args))

EDIT: A small addition: if you really need only keyword arguments of a function, you can use the func_defaults attribute to extract them:

def getValidKwargs(func, argsDict):
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    kwargsLen = len(func.func_defaults) # number of keyword arguments
    validKwargs = validArgs[-kwargsLen:] # because kwargs are last
    return dict((key, value) for key, value in argsDict.iteritems() 
                if key in validKwargs)

You could now call your function with known args, but extracted kwargs, e.g.:

func(param1, param2, **getValidKwargs(func, kwargsDict))

This assumes that func uses no *args or **kwargs magic in its signature.


回答 2

在Python 3.0中:

>>> import inspect
>>> import fileinput
>>> print(inspect.getfullargspec(fileinput.input))
FullArgSpec(args=['files', 'inplace', 'backup', 'bufsize', 'mode', 'openhook'],
varargs=None, varkw=None, defaults=(None, 0, '', 0, 'r', None), kwonlyargs=[], 
kwdefaults=None, annotations={})

In Python 3.0:

>>> import inspect
>>> import fileinput
>>> print(inspect.getfullargspec(fileinput.input))
FullArgSpec(args=['files', 'inplace', 'backup', 'bufsize', 'mode', 'openhook'],
varargs=None, varkw=None, defaults=(None, 0, '', 0, 'r', None), kwonlyargs=[], 
kwdefaults=None, annotations={})

回答 3

对于Python 3解决方案,您可以inspect.signature根据想要了解的参数种类使用和过滤。

以带有位置或关键字,仅关键字,var positional和var关键字参数的样本函数为例:

def spam(a, b=1, *args, c=2, **kwargs):
    print(a, b, args, c, kwargs)

您可以为其创建签名对象:

from inspect import signature
sig =  signature(spam)

然后使用列表推导进行过滤以找出所需的详细信息:

>>> # positional or keyword
>>> [p.name for p in sig.parameters.values() if p.kind == p.POSITIONAL_OR_KEYWORD]
['a', 'b']
>>> # keyword only
>>> [p.name for p in sig.parameters.values() if p.kind == p.KEYWORD_ONLY]
['c']

同样,对于使用p.VAR_POSITIONALvar关键字和var关键字与的变量VAR_KEYWORD

另外,您可以在if中添加一个子句,以通过检查是否p.default等于来检查是否存在默认值p.empty

For a Python 3 solution, you can use inspect.signature and filter according to the kind of parameters you’d like to know about.

Taking a sample function with positional or keyword, keyword-only, var positional and var keyword parameters:

def spam(a, b=1, *args, c=2, **kwargs):
    print(a, b, args, c, kwargs)

You can create a signature object for it:

from inspect import signature
sig =  signature(spam)

and then filter with a list comprehension to find out the details you need:

>>> # positional or keyword
>>> [p.name for p in sig.parameters.values() if p.kind == p.POSITIONAL_OR_KEYWORD]
['a', 'b']
>>> # keyword only
>>> [p.name for p in sig.parameters.values() if p.kind == p.KEYWORD_ONLY]
['c']

and, similarly, for var positionals using p.VAR_POSITIONAL and var keyword with VAR_KEYWORD.

In addition, you can add a clause to the if to check if a default value exists by checking if p.default equals p.empty.


回答 4

扩展DzinX的答案:

argnames = example.func_code.co_varnames[:func.func_code.co_argcount]
args = dict((key, val) for key,val in d_args.iteritems() if key in argnames)
example(**args)

Extending DzinX’s answer:

argnames = example.func_code.co_varnames[:func.func_code.co_argcount]
args = dict((key, val) for key,val in d_args.iteritems() if key in argnames)
example(**args)