Passing to a function -- object and method names (or references)

[Midas]

Greetings everyone,

I'm including code, for cut/paste, to better explain my question. I create a Car object then I call some of its methods. No problem. Then I try to pass, to a function, the name of the Car object and the name of one of its methods. I found one way to get this to work but can someone show me a more orthodox way? Is there a way using references to the object and somehow the method?

Python 2.2.2 (#37, Oct 14 2002, 17:02:34) [MSC 32 bit (Intel)] on win32
IDLE 0.8

class Car:
def __init__(self):
self.milespergallon=25.0
self.gas=20
self.travelled=0
def drive(self, miles):
self.travelled=self.travelled+miles
self.gas=self.gas-(miles/self.milespergallon)


# a test
16.0

# Next, trying to pass object and method "references", to someFuncA

def someFuncA(objArg1,strArg2):
print carObjB.strMethB
Traceback (most recent call last):
File "<pyshell#16>", line 1, in ?
someFuncA(carObjB,strMethB)
File "<pyshell#15>", line 2, in someFuncA
print carObjB.strMethB
AttributeError: Car instance has no attribute 'strMethB'

# Next, trying to pass object and method "references", to someFuncB

def someFuncB(strArg1,strArg2):
e = "print " + strArg1 + "." + strArg2
exec e
16.0

# That worked but is there a more orthodox way to pass these "references"?

 

[Peter Otten]

I'm including code, for cut/paste, to better explain my question. I create
a Car object then I call some of its methods. No problem. Then I try to
pass, to a function, the name of the Car object and the name of one of its
methods. I found one way to get this to work but can someone show me a
more orthodox way? Is there a way using references to the object and
somehow the method?

You can access attributes with the getattr() builtin.

carGas = getattr(car, "gas")

You can refer to a method either directly

fun = car.drive
fun(123)

or via getattr():

fun = getattr(car, "drive")
fun(321)

I've modified your example to demonstrate this.

class NoGas(Exception): pass

class Car:
def __init__(self):
self.milespergallon = 25.0
self.gas = 20
self.travelled = 0
def drive(self, miles):
newGas = self.gas - miles/self.milespergallon
if newGas < 0:
self.travelled += self.milespergallon*self.gas
self.gas = 0
raise NoGas("%s miles travelled. No more gas" %
self.travelled)
self.travelled += miles
self.gas = newGas


def printAttr(obj, attrname):
"Demo for accessing an attribute by its name"
print attrname, "=", getattr(obj, attrname)

def callMethod(obj, methodname, *args):
""" Demo for calling a method determined by its name.
An arbitrary number of arguments is just passed
through to the method.
"""
method = getattr(obj, methodname)
print "calling", methodname
method(*args)

def callMethod2(method, *args):
""" Demo for calling a method reference.
The method is is generated by obj.method in the
calling code. Of course you can pass function
references as well
"""
method(*args)

def someFunction():
print "Welcome to the Python Motorshow"

callMethod2(someFunction)
car = Car()
printAttr(car, "gas")
callMethod(car, "drive", 10)
printAttr(car, "gas")
while True:
printAttr(car, "travelled")
callMethod2(car.drive, 100)

Note that my car will not travel as far as yours as it is less satisfied
with negative amounts of gas

Peter

 

[Midas]

You can access attributes with the getattr() builtin.

carGas = getattr(car, "gas")

You can refer to a method either directly

fun = car.drive
fun(123)

or via getattr():

fun = getattr(car, "drive")
fun(321)

I've modified your example to demonstrate this.

Thank you very much, Peter! It works nicely!

Midas