Using setattr() in python
Solution 1
You are setting self.name
to the string "get_thing"
, not the function get_thing
.
If you want self.name
to be a function, then you should set it to one:
setattr(self, 'name', self.get_thing)
However, that's completely unnecessary for your other code, because you could just call it directly:
value_returned = self.get_thing()
Solution 2
The Python docs say all that needs to be said, as far as I can see.
setattr
(object, name, value)This is the counterpart of
getattr()
. The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example,setattr(x, 'foobar', 123)
is equivalent tox.foobar = 123
.
Solution 3
Setattr: We use setattr to add an attribute to our class instance. We pass the class instance, the attribute name, and the value. and with getattr we retrive these values
For example
Employee = type("Employee", (object,), dict())
employee = Employee()
# Set salary to 1000
setattr(employee,"salary", 1000 )
# Get the Salary
value = getattr(employee, "salary")
print(value)
Solution 4
To add to the other answers, a common use case I have found for setattr()
is when using configs. It is common to parse configs from a file (.ini file or whatever) into a dictionary. So you end up with something like:
configs = {'memory': 2.5, 'colour': 'red', 'charge': 0, ... }
If you want to then assign these configs to a class to be stored and passed around, you could do simple assignment:
MyClass.memory = configs['memory']
MyClass.colour = configs['colour']
MyClass.charge = configs['charge']
...
However, it is much easier and less verbose to loop over the configs, and setattr()
like so:
for name, val in configs.items():
setattr(MyClass, name, val)
As long as your dictionary keys have the proper names, this works very well and is nice and tidy.
*Note, the dict keys need to be strings as they will be the class object names.
Solution 5
Suppose you want to give attributes to an instance which was previously not written in code.
The setattr()
does just that.
It takes the instance of the class self
and key and value to set.
class Example:
def __init__(self, **kwargs):
for key, value in kwargs.items():
setattr(self, key, value)
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bounce
Updated on September 04, 2021Comments
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bounce almost 3 years
I am looking for someone to explain the basics of how to use, and not use
setattr()
.My problem arose trying to use one class method/function to return data that is then put in another method/function. Perhaps a simpler approach would be much better in this case, but I'm trying to understand how classes work/are used. This problem seems to hinge on
setattr()
, and this is my attempt to make a fairly simple use of this.Though it's not quite the same problem, I was following Python The Hard Way, ex42—the
while
loop @ lines 18-41.I tried writing an
\__init__()
, and usinggetattr()
instead, thinking perhaps something needed to be in the class' namespace, but this doesn't seem to help.#! /bin/python2.6 class HolyGrail(object): def __init__(self): self.start = 'start_at_init' # function definition in question: # TypeError: 'str' object is not callable def run_it(self): start = setattr(self, 'name', 'get_thing') start = self.name # Something wrong here? value_returned = start() #I believe this == self.get_thing() use_it(value_returned) """ # alternate function definitions # NameError: global name 'start' is not defined def __init__(self): self.start = 'get_thing' def run_it(self): go_do_it = getattr(self, start) first_output = go_do_it() use_it(first_output) """ def get_thing(self): return "The Knights Who Say ... Ni!" def use_it(self, x): print x print "We want a shrubbery!" my_instance = HolyGrail() my_instance.run_it()
@Karl Knechtel, @Amber , @Chris Morgan thanks for your help.
I think I can now explain my own answer! This required a better grasp of self as an object for me. It's an instance name that gets tagged up with stuff like attributes.
The class could be a Town, and then.
getattr
looks for a house using it's name so you are ready to call on it soon, and comes up with a different place if you don't find the house --Withgetattr
a 'name' exists, and you go find it. Makes the step from one function to another dynamic As a bonus you may have a default value, useful to get a fallback default method--connection failed or something?setattr
builds a house and gives it a name so you can call in on it later. You could potentially rebuild this house, or go to a particular place if you are unable to find it. --setattr
makes an attribute name and gives, or changes it's value, to be called on later Perhaps a user turns sound off, then future methods don't output any audio.I could have written my function a number of ways, but there's no need to change any attributes:
def run_it(self): yo = getattr(self, 'get_thing') answer = yo() setattr(self, 'deal_accepted', self.use_it) #really ott no = getattr(self, 'deal_accepted') no(answer)
Properly corrected code:
def run_it(self): value_returned = self.get_thing() self.use_it(value_returned)
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Karl Knechtel over 12 years
setattr
is for situations where you can't do it directly. As a beginner, these should be nonexistent.setattr(self, 'name', 'get_thing')
means exactly the same asself.name = 'get_thing'
. -
user2471801 over 12 yearsplease don't use docstring notation for a block comment. use # instead. python.org/dev/peps/pep-0008
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Chris Morgan over 12 years@monkut: triple-quoted string, not docstring; "docstring" just refers to a string which is the first statement in a module, class or function definition (or the
__doc__
attribute of an object). -
user2471801 over 12 yearsyou're right. In any case don't use strings as comments.
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Haleemur Ali about 9 years@monkut, why shouldn't one use strings as comments?
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user2471801 about 9 years@Haleemur Ali Generally speaking comments are intended to provide clarification of code actions, and are not intended as output of the program. A commented line is not analyzed by the interpreter, so it does not affect run time behavior. Creating strings consumes memory and processing time.
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Guimoute almost 3 years@HaleemurAli And they are not colored the same by your IDE (hopefully), so filling the code with unassigned strings instead of real comments is visual pollution.
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bounce over 12 yearsI see! Got it! You just have to tell python where the function is, self! (so getattr's value would also also be self.function). Thank you Amber!
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Nostalg.io over 9 yearsThanks. Question, if dot notation is so much more succinct, then why use setattr at all?
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Chris Morgan over 9 years@halfnibble: it’s useful when
name
is not a string literal, when the attribute you are setting is dynamically determined, e.g. in some forms of metaprogramming. -
e.thompsy about 9 years@halfnibble: An example:
for key in data: value = request.DATA.get(key) setattr(obj, key, value)
<-- iterates through all provided POST data and assigns to obj using key as the field. -
denis about 8 years@Chris Morgan, how do some objects disallow
setattr
? E.g.x = numpy.ones(3); setattr( x, "name", "xname" )
--> AttributeError ? (I'll ask a new question if you like) -
Chris Morgan about 8 years@denis:
setattr
doesn’t necessarily permit you to set an attribute of any name on an object. If the type uses__dict__
, it does, but a type produced in C with a known set of attributes may not, or from Python code a type which defines__slots__
. In such cases, you can usesetattr
to set acceptable attributes, but others will produce anAttributeError
. -
denis about 8 yearsThanks. I just wanted numpy arrays with names, but seems you'd have to subclass ndarray, don't try this at home
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zml almost 7 yearsIs there a way to access the attribute's name
'name'
inside theget_thing()
function ? -
Eduardo Pignatelli almost 6 yearsIs there a python architectural reason the method doesn't return a boolean of success?
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Chris Morgan almost 6 years@EduardoPignatelli: returning a bool is for cases where True and False are both normal outcomes. This is not the case when calling setattr(); there, failure is decidedly abnormal, strongly indicating a programming error, and raising an exception in case of failure and returning nothing in other cases is the appropriate course of action in Python. This also makes it so that setattr() is precisely equivalent to the corresponding assignment statement, which produces nothing and can raise an AttributeError.
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bjd2385 almost 5 yearsAFAIK not unless you, again,
setattr
it on the container function.