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calling a function from string

hi,

i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?

Thanks
james

Oct 22 '07 #1
10 2896
On 10/22/07, james_027 <ca********@gmail.comwrote:
hi,

i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?
you could use getattr:

function_name = 'time' # this is a string
module_name = 'time' # this is a string, too

my_function = getattr(module_name, function_name) # this is the
function object,
# equivalent to my_function = time.time
my_function() # This is the function call, equivalent to time.time()

bye
francesco
Oct 22 '07 #2
i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?
Use 'eval' in one of the following fashions:

a_string_1 = 'datetime.' + 'today'
a_string_2 = 'datetime.' + 'today()'

eval(a_string_1)()
eval(a_string_2)
Trent.
Oct 22 '07 #3
Trent Nelson napisa³(a):
>i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?

Use 'eval' in one of the following fashions:

a_string_1 = 'datetime.' + 'today'
a_string_2 = 'datetime.' + 'today()'

eval(a_string_1)()
eval(a_string_2)
Do not use eval(). Not only it's deprecated, it's also unsafe.

--
Jarek Zgoda
Skype: jzgoda | GTalk: zg***@jabber.aster.pl | voice: +48228430101

"We read Knuth so you don't have to." (Tim Peters)
Oct 22 '07 #4
On Oct 22, 4:41 am, "Francesco Guerrieri" <f.guerri...@gmail.com>
wrote:
On 10/22/07, james_027 <cai.hai...@gmail.comwrote:
hi,
i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.
for example
a_string = 'datetime.' + 'today()'
how could I call a_string as function?

you could use getattr:

function_name = 'time' # this is a string
module_name = 'time' # this is a string, too

my_function = getattr(module_name, function_name) # this is the
function object,
# equivalent to my_function = time.time

Not quite.

============================================
>>function_name = 'time' # this is a string
module_name = 'time' # this is a string, too
my_function = getattr(module_name, function_name)
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
my_function = getattr(module_name, function_name)
AttributeError: 'str' object has no attribute 'time'
============================================

It's actually equivalent to:

============================================
>>"time".time
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
"time".time
AttributeError: 'str' object has no attribute 'time'
============================================

Oct 22 '07 #5
On Oct 22, 5:46 am, Jarek Zgoda <jzg...@o2.usun.plwrote:
Do not use eval(). Not only it's deprecated, it's also unsafe.
I don't think it's deprecated; it doesn't say so:
http://docs.python.org/lib/built-in-funcs.html#l2h-25

Oct 22 '07 #6
james_027 a écrit :
hi,

i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?
The obvious answer is to use eval or exec, but it's 99.99 times out of
100 the wrong solution.

Better solutions usually rely on Python's introspection features -
mostly globals(), locals(), sys.modules, and of course getattr().
Oct 22 '07 #7
Jarek Zgoda a écrit :
Trent Nelson napisa³(a):
>>i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?
Use 'eval' in one of the following fashions:

a_string_1 = 'datetime.' + 'today'
a_string_2 = 'datetime.' + 'today()'

eval(a_string_1)()
eval(a_string_2)

Do not use eval(). Not only it's deprecated,
Chapter and verse ???
it's also unsafe.
it's *potentially* unsafe. As long as the eval'd code comes from a
trusted source, there should be no security problem.

I agree that eval is usually not the solution, but mainly because Python
has far better (wrt/ readability and maintainance) options for this kind
of things.

Oct 22 '07 #8
>>exec("import datetime") ; exec("x = datetime." + "date." + "today()")
>>print x
2007-10-22


james_027 wrote:
hi,

i have a function that I could like to call, but to make it more
dynamic I am constructing a string first that could equivalent to the
name of the function I wish to call. how could I do that? the string
could might include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?

Thanks
james


--
Shane Geiger
IT Director
National Council on Economic Education
sg*****@ncee.net | 402-438-8958 | http://www.ncee.net

Leading the Campaign for Economic and Financial Literacy
Oct 22 '07 #9
On Mon, 22 Oct 2007 08:54:02 +0000, james_027 wrote:
hi,

i have a function that I could like to call, but to make it more dynamic
I am constructing a string first that could equivalent to the name of
the function I wish to call.
That is not the right solution to dynamic functions. There is a much
better way.

how could I do that? the string could might
include name of the module.

for example

a_string = 'datetime.' + 'today()'

how could I call a_string as function?
Others have suggested eval() and exec. Both will work, but have MAJOR
security implications.

The right way to work with "dynamic functions" is to remember that Python
treats functions as first-class objects just like strings and ints and
lists. Here's a simple example:

Suppose I have a function that takes a string and converts it to another
object type.

def converter(x, convert_to):
if convert_to == 'int':
return int(x)
elif convert_to == 'float':
return float(x)
elif convert_to == 'list':
return list(x)
else:
raise ValueError("don't know that type")

and then use the function like this:

my_float = converter('12.345', 'float')
That's the wrong way to do it. This is the right way:

def converter(x, convert_to):
return convert_to(x)

my_float = converter('12.345', float)

See the subtle difference?

'float' is a string, and it has no special meaning.

float() with brackets says "call the function float".

float without brackets *is* the function float. You can pass it around
like any other object (strings, lists, ints, etc.) and call it later.

Try this example:

import datetime, time
functions = [int, float, datetime.time, time.time]
for f in functions:
print f()

--
Steven
Oct 22 '07 #10
On Mon, 22 Oct 2007 23:16:38 +0000, Steven D'Aprano wrote:
>how could I call a_string as function?

Others have suggested eval() and exec. Both will work, but have MAJOR
security implications.
Oh, and they are seriously slower too.
>>import timeit
timeit.Timer('f("2.3")',
.... 'f = float').repeat() # time using 1st class function
[1.7266130447387695, 0.97645401954650879, 0.97271394729614258]
>>timeit.Timer('eval("float")("2.3")'
.... ).repeat() # time using eval and a string
[20.628923177719116, 19.70452094078064, 19.783280849456787]
--
Steven.
Oct 22 '07 #11

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