Nov 10, 2014

How do I use strings to call functions/methods?

There are various techniques.

* The best is to use a dictionary that maps strings to functions. The primary advantage of this technique is that the strings do not need to match the names of the functions. This is also the primary technique used to emulate a case construct:

def a():

def b():

dispatch = {'go': a, 'stop': b} # Note lack of parens for funcs

dispatch[get_input()]() # Note trailing parens to call function
Use the built-in function getattr():

import foo
getattr(foo, 'bar')()

Note that getattr() works on any object, including classes, class instances, modules, and so on.

This is used in several places in the standard library, like this:

class Foo:
def do_foo(self):
def do_bar(self):

f = getattr(foo_instance, 'do_' + opname)

Use locals() or eval() to resolve the function name:

def myFunc():
print "hello"

fname = "myFunc"

f = locals()[fname]

f = eval(fname)

Note: Using eval() is slow and dangerous. If you don't have absolute control over the contents of the string, someone could pass a string that resulted in an arbitrary function being executed.
Is there an equivalent to Perl's chomp() for removing trailing newlines from strings? 
Starting with Python 2.2, you can use S.rstrip("\r\n") to remove all occurences of any line terminator from the end of the string S without removing other trailing whitespace. If the string S represents more than one line, with several empty lines at the end, the line terminators for all the blank lines will be removed:

>>> lines = ("line 1 \r\n"
... "\r\n"
... "\r\n")
>>> lines.rstrip("\n\r")
"line 1 "

Since this is typically only desired when reading text one line at a time, using S.rstrip() this way works well.

For older versions of Python, There are two partial substitutes:

* If you want to remove all trailing whitespace, use the rstrip() method of string objects. This removes all trailing whitespace, not just a single newline.
* Otherwise, if there is only one line in the string S, use S.splitlines()[0].


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