'How to get method parameter names?
Given the Python function:
def a_method(arg1, arg2):
pass
How can I extract the number and names of the arguments. I.e., given that I have a reference to func, I want the func.[something] to return ("arg1", "arg2").
The usage scenario for this is that I have a decorator, and I wish to use the method arguments in the same order that they appear for the actual function as a key. I.e., how would the decorator look that printed "a,b" when I call a_method("a", "b")?
Solution 1:[1]
Take a look at the inspect module - this will do the inspection of the various code object properties for you.
>>> inspect.getfullargspec(a_method)
(['arg1', 'arg2'], None, None, None)
The other results are the name of the *args and **kwargs variables, and the defaults provided. ie.
>>> def foo(a, b, c=4, *arglist, **keywords): pass
>>> inspect.getfullargspec(foo)
(['a', 'b', 'c'], 'arglist', 'keywords', (4,))
Note that some callables may not be introspectable in certain implementations of Python. For Example, in CPython, some built-in functions defined in C provide no metadata about their arguments. As a result, you will get a ValueError if you use inspect.getfullargspec() on a built-in function.
Since Python 3.3, you can use inspect.signature() to see the call signature of a callable object:
>>> inspect.signature(foo)
<Signature (a, b, c=4, *arglist, **keywords)>
Solution 2:[2]
In CPython, the number of arguments is
a_method.func_code.co_argcount
and their names are in the beginning of
a_method.func_code.co_varnames
These are implementation details of CPython, so this probably does not work in other implementations of Python, such as IronPython and Jython.
One portable way to admit "pass-through" arguments is to define your function with the signature func(*args, **kwargs). This is used a lot in e.g. matplotlib, where the outer API layer passes lots of keyword arguments to the lower-level API.
Solution 3:[3]
The Python 3 version is:
def _get_args_dict(fn, args, kwargs):
args_names = fn.__code__.co_varnames[:fn.__code__.co_argcount]
return {**dict(zip(args_names, args)), **kwargs}
The method returns a dictionary containing both args and kwargs.
Solution 4:[4]
In a decorator method, you can list arguments of the original method in this way:
import inspect, itertools
def my_decorator():
def decorator(f):
def wrapper(*args, **kwargs):
# if you want arguments names as a list:
args_name = inspect.getargspec(f)[0]
print(args_name)
# if you want names and values as a dictionary:
args_dict = dict(itertools.izip(args_name, args))
print(args_dict)
# if you want values as a list:
args_values = args_dict.values()
print(args_values)
If the **kwargs are important for you, then it will be a bit complicated:
def wrapper(*args, **kwargs):
args_name = list(OrderedDict.fromkeys(inspect.getargspec(f)[0] + kwargs.keys()))
args_dict = OrderedDict(list(itertools.izip(args_name, args)) + list(kwargs.iteritems()))
args_values = args_dict.values()
Example:
@my_decorator()
def my_function(x, y, z=3):
pass
my_function(1, y=2, z=3, w=0)
# prints:
# ['x', 'y', 'z', 'w']
# {'y': 2, 'x': 1, 'z': 3, 'w': 0}
# [1, 2, 3, 0]
Solution 5:[5]
I think what you're looking for is the locals method -
In [6]: def test(a, b):print locals()
...:
In [7]: test(1,2)
{'a': 1, 'b': 2}
Solution 6:[6]
Python 3.5+:
DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()
So previously:
func_args = inspect.getargspec(function).args
Now:
func_args = list(inspect.signature(function).parameters.keys())
To test:
'arg' in list(inspect.signature(function).parameters.keys())
Given that we have function 'function' which takes argument 'arg', this will evaluate as True, otherwise as False.
Example from the Python console:
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 07:18:10) [MSC v.1900 32 bit (Intel)] on win32
>>> import inspect
>>> 'iterable' in list(inspect.signature(sum).parameters.keys())
True
Solution 7:[7]
Here is something I think will work for what you want, using a decorator.
class LogWrappedFunction(object):
def __init__(self, function):
self.function = function
def logAndCall(self, *arguments, **namedArguments):
print "Calling %s with arguments %s and named arguments %s" %\
(self.function.func_name, arguments, namedArguments)
self.function.__call__(*arguments, **namedArguments)
def logwrap(function):
return LogWrappedFunction(function).logAndCall
@logwrap
def doSomething(spam, eggs, foo, bar):
print "Doing something totally awesome with %s and %s." % (spam, eggs)
doSomething("beans","rice", foo="wiggity", bar="wack")
Run it, it will yield the following output:
C:\scripts>python decoratorExample.py
Calling doSomething with arguments ('beans', 'rice') and named arguments {'foo':
'wiggity', 'bar': 'wack'}
Doing something totally awesome with beans and rice.
Solution 8:[8]
In Python 3.+ with the Signature object at hand, an easy way to get a mapping between argument names to values, is using the Signature's bind() method!
For example, here is a decorator for printing a map like that:
import inspect
def decorator(f):
def wrapper(*args, **kwargs):
bound_args = inspect.signature(f).bind(*args, **kwargs)
bound_args.apply_defaults()
print(dict(bound_args.arguments))
return f(*args, **kwargs)
return wrapper
@decorator
def foo(x, y, param_with_default="bars", **kwargs):
pass
foo(1, 2, extra="baz")
# This will print: {'kwargs': {'extra': 'baz'}, 'param_with_default': 'bars', 'y': 2, 'x': 1}
Solution 9:[9]
Here is another way to get the function parameters without using any module.
def get_parameters(func):
keys = func.__code__.co_varnames[:func.__code__.co_argcount][::-1]
sorter = {j: i for i, j in enumerate(keys[::-1])}
values = func.__defaults__[::-1]
kwargs = {i: j for i, j in zip(keys, values)}
sorted_args = tuple(
sorted([i for i in keys if i not in kwargs], key=sorter.get)
)
sorted_kwargs = {
i: kwargs[i] for i in sorted(kwargs.keys(), key=sorter.get)
}
return sorted_args, sorted_kwargs
def f(a, b, c="hello", d="world"): var = a
print(get_parameters(f))
Output:
(('a', 'b'), {'c': 'hello', 'd': 'world'})
Solution 10:[10]
inspect.signature is very slow. Fastest way is
def f(a, b=1, *args, c, d=1, **kwargs):
pass
f_code = f.__code__
f_code.co_varnames[:f_code.co_argcount + f_code.co_kwonlyargcount] # ('a', 'b', 'c', 'd')
Solution 11:[11]
Returns a list of argument names, takes care of partials and regular functions:
def get_func_args(f):
if hasattr(f, 'args'):
return f.args
else:
return list(inspect.signature(f).parameters)
Solution 12:[12]
Update for Brian's answer:
If a function in Python 3 has keyword-only arguments, then you need to use inspect.getfullargspec:
def yay(a, b=10, *, c=20, d=30):
pass
inspect.getfullargspec(yay)
yields this:
FullArgSpec(args=['a', 'b'], varargs=None, varkw=None, defaults=(10,), kwonlyargs=['c', 'd'], kwonlydefaults={'c': 20, 'd': 30}, annotations={})
Solution 13:[13]
In python 3, below is to make *args and **kwargs into a dict (use OrderedDict for python < 3.6 to maintain dict orders):
from functools import wraps
def display_param(func):
@wraps(func)
def wrapper(*args, **kwargs):
param = inspect.signature(func).parameters
all_param = {
k: args[n] if n < len(args) else v.default
for n, (k, v) in enumerate(param.items()) if k != 'kwargs'
}
all_param .update(kwargs)
print(all_param)
return func(**all_param)
return wrapper
Solution 14:[14]
Simple easy to read answer as of python 3.0 onwards:
import inspect
args_names = inspect.signature(function).parameters.keys()
args_dict = {
**dict(zip(args_names, args)),
**kwargs,
}
Solution 15:[15]
To update a little bit Brian's answer, there is now a nice backport of inspect.signature that you can use in older python versions: funcsigs.
So my personal preference would go for
try: # python 3.3+
from inspect import signature
except ImportError:
from funcsigs import signature
def aMethod(arg1, arg2):
pass
sig = signature(aMethod)
print(sig)
For fun, if you're interested in playing with Signature objects and even creating functions with random signatures dynamically you can have a look at my makefun project.
Solution 16:[16]
I was googling to find how to print function name and supplied arguments for an assignment I had to create a decorator to print them and I used this:
def print_func_name_and_args(func):
def wrapper(*args, **kwargs):
print(f"Function name: '{func.__name__}' supplied args: '{args}'")
func(args[0], args[1], args[2])
return wrapper
@print_func_name_and_args
def my_function(n1, n2, n3):
print(n1 * n2 * n3)
my_function(1, 2, 3)
#Function name: 'my_function' supplied args: '(1, 2, 3)'
Solution 17:[17]
Is it possible to use inspect API to read constant argument value -1 from the lambda func fun in the code below?
def my_func(v, axis):
pass
fun = lambda v: my_func(v, axis=-1)
Sources
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