Newbie: How can I use a string value for a keyword argument?

[Doug Morse]

Hi,

My apologies for troubling for what is probably an easy question... it's just
that can't seem to find an answer to this anywhere (Googling, pydocs, etc.)...

I have a class method, MyClass.foo(), that takes keyword arguments. For
example, I can say:

x = MyClass()
x.foo(trials=32)

Works just fine.

What I need to be able to do is call foo() with a string value specifying the
keyword (or both the keyword and value would be fine), something along the
lines of:

x = MyClass()
y = 'trials=32'
x.foo(y) # doesn't work

or

x.MyClass()
y = 'trials'
x.foo(y = 32) # does the "wrong" thing

Surely there's some way to use a string's value as the key for making a method
call with a keyword argument?

Just for completeness, my goal is simply to read a bunch of key/value pairs
from an INI file (using ConfigObj) and then use those key/value pairs to set a
(3rd party) object's parameters, which must be done with a call along the
lines of "instance.set(key=value)". Obviously, I could create a huge if..elif
statement along the lines of "if y = 'trials': x.foo(trials=32); elif y =
'speed': x.foo(speed=12);" etc., but then the statement has to be maintained
every time a new parameter is added/changed etc. Plus, such a solution seems
to me grossly inelegant and un-Pythonic.

Thanks in advance for any and all assistance!

Doug

 

[John Machin]

Hi,

My apologies for troubling for what is probably an easy question... it's just
that can't seem to find an answer to this anywhere (Googling, pydocs, etc.)...

I have a class method, MyClass.foo(), that takes keyword arguments. For
example, I can say:

x = MyClass()
x.foo(trials=32)

Works just fine.

What I need to be able to do is call foo() with a string value specifying the
keyword (or both the keyword and value would be fine), something along the
lines of:

x = MyClass()
y = 'trials=32'
x.foo(y) # doesn't work

or

x.MyClass()
y = 'trials'
x.foo(y = 32) # does the "wrong" thing

Surely there's some way to use a string's value as the key for making a method
call with a keyword argument?

Just for completeness, my goal is simply to read a bunch of key/value pairs
from an INI file (using ConfigObj) and then use those key/value pairs to set a
(3rd party) object's parameters, which must be done with a call along the
lines of "instance.set(key=value)". Obviously, I could create a huge if..elif
statement along the lines of "if y = 'trials': x.foo(trials=32); elif y =
'speed': x.foo(speed=12);" etc., but then the statement has to be maintained
every time a new parameter is added/changed etc. Plus, such a solution seems
to me grossly inelegant and un-Pythonic.

I'm not quite sure what foo() is really supposed to do ... however the
built-in function setattr is your friend. Assuming that ini_dict
contains what you have scraped out of your .ini file, you can do:
x = MyCLass()
for key, value in ini_dict.items():
setattr(x, key, value)
You may prefer (I would) to do it inside the class, and maybe do some
checking/validation:
class MyClass(object):
def load(self, adict):
for k, v in adict.items():
# do checking here
setattr(self, k, v)
# much later
x = MyClass()
x.load(ini_dict)

HTH,
John

 

[bockman]

Hi,

My apologies for troubling for what is probably an easy question... it's just
that can't seem to find an answer to this anywhere (Googling, pydocs, etc.)...

I have a class method, MyClass.foo(), that takes keyword arguments.  For
example, I can say:

x = MyClass()
x.foo(trials=32)

Works just fine.

What I need to be able to do is call foo() with a string value specifying the
keyword (or both the keyword and value would be fine), something along the
lines of:

x = MyClass()
y = 'trials=32'
x.foo(y)        # doesn't work

or

x.MyClass()
y = 'trials'
x.foo(y = 32)   # does the "wrong" thing

Try this:
y='trials'
x.foo( **{y:32} )

Ciao

 

[Bruno Desthuilliers]

Doug Morse a écrit :

Hi,

My apologies for troubling for what is probably an easy question... it's just
that can't seem to find an answer to this anywhere (Googling, pydocs, etc.)...

I have a class method, MyClass.foo(), that takes keyword arguments. For
example, I can say:

x = MyClass()
x.foo(trials=32)

Works just fine.

What I need to be able to do is call foo() with a string value specifying the
keyword (or both the keyword and value would be fine), something along the
lines of:

x = MyClass()
y = 'trials=32'
x.foo(y) # doesn't work

You want something like:

x.foo(**{'trials':32})

Just for completeness, my goal is simply to read a bunch of key/value pairs
from an INI file (using ConfigObj)

ConfigObj being a subclass of dict, you should be able to use it
directly, ie:

x.foo(**my_config)

If just want to pass a section of the ConfigObj, it should work just the
same.

HTH

 

[Doug Morse]

I'm not quite sure what foo() is really supposed to do ... however the
built-in function setattr is your friend. Assuming that ini_dict
contains what you have scraped out of your .ini file, you can do:
x = MyCLass()
for key, value in ini_dict.items():
setattr(x, key, value)
You may prefer (I would) to do it inside the class, and maybe do some
checking/validation:
class MyClass(object):
def load(self, adict):
for k, v in adict.items():
# do checking here
setattr(self, k, v)
# much later
x = MyClass()
x.load(ini_dict)

HTH,
John

Hi John,

Your response is most helpful and informative -- thanks!

I don't think that setattr() is exactly what I need, though, as foo() doesn't
actually create or update its instance attributes. What I need to be able to
do is call foo() specifying keyword arguments not directly but viz a viz
another variable or variables that contain the keywords and values.

I'm pretty sure I just found the solution, which is to use the **-operator on
a dictionary. Actually, ConfigObj (the INI file reader) subclasses from
__builtin__.dict (i.e., the class/object *is* a dictionary), so the following
seems to work perfectly:

x.foo(**config)

This sends ALL the key/value pairs in config as keyword/value pairs to foo(),
which is exactly what I need.

Just FYI, I located this solution via Google shortly after posting, so I have
sent a cancel request on my original post.

Thanks again!
Doug

 

[Gary Herron]

Hi,

My apologies for troubling for what is probably an easy question... it's just
that can't seem to find an answer to this anywhere (Googling, pydocs, etc.)...

I have a class method, MyClass.foo(), that takes keyword arguments. For
example, I can say:

x = MyClass()
x.foo(trials=32)

Works just fine.

What I need to be able to do is call foo() with a string value specifying the
keyword (or both the keyword and value would be fine), something along the
lines of:

x = MyClass()
y = 'trials=32'
x.foo(y) # doesn't work

Keyword args are represented during the calling process as a
dictionary, You can create the dictionary yourself, and slip it into
the calling arguments with a ** notation:

kw = {somestring:32}
x.foo(**kw)

Gary Herron