Passing method to a function in Python

Question

I'm trying to pass a method, specifically a method of the string class, to a function which will run it. My code looks something like:

def foo (list):
 for in_str in list[0]:
 print(in_str.list[1]())
# format of list
list = [
 [["Hello world!", "helloworld"], str.isalpha]
]

The desired operation of the function would be to invoke the isalpha method on the string in_str then print the result. This doesn't work because in_str obviously doesn't have the attribute list, but I want to know how to make list[1] reference the method.

Any suggestions?

Thanks!

Answer 1

def foo (l):
 for in_str in l[0]:
 print(l[1](in_str))
# format of list
l = [["Hello world!", "helloworld"], str.isalpha]
print(foo(l))

You need to pass a string to str.isalpha, also you have an extra pair of brackets in the list.

In [2]: foo(l)
False
True

If you just want to pass a function then pass it as a parameter and just pass a list of strings:

def foo(l, func):
 for in_str in l[0]:
 print(func(in_str))
l = ["Hello world!", "helloworld"]
print(foo(l,str.isalpha))

A nicer way may be to use map to map the func to each string:

def foo(l,func):
 return map(func,l) # list(map(...) python 3
print(foo(l,str.isalpha))

• Bhargav Rao: Yep! That's the perfect answer. But, you say also you have an extra pair of brackets in the list, What if the OP calls the function as print(foo(l[0]))
• Padraic Cunningham: @BhargavRao, iI think it was a typo by the OP, as using l[0], in_str `would be a list. If not we can do a little change
• Jason: I'm not sure if I'm wording this properly, but what I desire the function to do is invoke in_str.isalpha() by passing it as a parameter. My actual program is much larger and has many different methods to pass, but need to be loaded by a list.
• Padraic Cunningham: @SuperUser320, you can simply pass it as a parameter
• Jason: Hmm. I think I have larger issues. When I run that in the python shell it works, yet in my program it returns [] instead of "True" or "False"

Answer 2

Either you call the function isalpha with a string as an argument:

str.isalpha('Hello World!")

or as an object oriented approach:

'Hello World'.isalpha()

In your case we also need to correct some indices. I also changed the variable name list because it shadows the built-in list function.

def foo(anestedlist):
 for a_str in anestedlist[0][0]:
 print(anestedlist[0][1](a_str))
# format of list
anestedlist = [[["Hello world!", "helloworld"], str.isalpha]]

and

foo(anestedlist)

prints

False
True

The important part here is

print(anestedlist[0][1](a_str))

which translates to str.isalpha(a_str), i.e. passing a string as an argument.