Calling a method with a string and input()

Question

I'm brushing up on my Python and I'm a little confused about something, the following code does not work as intended:

def a():
    print "called a"

def b():
    print "called b"

dispatch = {'go':a, 'stop':b}
dispatch[input()]()

When I type the word go into the console, I get "NameError: name 'go' is not defined", but when I type 'go' (with the quotes) it works fine. Is input() not returning a string? And if not, then shouldn't using str() convert the input to a string?

When I change the code to:

dispatch[(str(input())]()

I still get the same behaviour.

note: I'm using Python2.7 if it makes a difference.

Sorry if this is obvious, it's been a few years since I've used Python!

TerryA · Accepted Answer · 2013-09-12 08:53:58Z

3

input() in Python 2.7 is equivalent to:

eval(raw_input())

Hence, you should use raw_input() in python 2.7 to avoid such errors, as it will not try to evaluate the input given.

Because you add in quotation marks, Python interprets this as a string.

Also, note that raw_input() will return a string, so there will be no point in calling str() around it.

Note that in Python 3, input() acts like raw_input() did in Python 2.7

edited Sep 12, 2013 at 8:53

answered Sep 12, 2013 at 8:47

TerryA

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3 Comments

Nick Boudreau Over a year ago

I read a list of the more 'notable' changes between 2.X and 3.X but this one wasn't listed, thanks!

TerryA Over a year ago

@NickBoudreau Hmm, where abouts was this? There's a great list at the docs if you wish to check it out :)

Nick Boudreau Over a year ago

I can't remember the site, it was just someone's blog I think, but I wrote down the changes they listed: strings are now unicode, print is a function not a statement, range is removed and replaced with xrange, division of integers results in a float and all classes are now new-style. Thanks for the link, very useful!

svk · Accepted Answer · 2013-09-12 08:47:31Z

2

You want raw_input(). input() evaluates the user's input as Python code.

answered Sep 12, 2013 at 8:47

svk

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Comments

Kaidjin · Accepted Answer · 2013-09-12 08:52:17Z

1

Use raw_input(). input() is equivalent to eval(raw_input()), so when you type go without quotes, python tries to eval go and fails. With the quotes, it eval's the string (ie. returns it) which will work to index your dict.

raw_input will work in every case.

answered Sep 12, 2013 at 8:52

Kaidjin

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Comments

Paco · Accepted Answer · 2013-09-12 09:11:09Z

1

You can also use this:

def go():
    print "called a"


def stop():
    print "called b"

input()()  # Equivalent to eval(raw_input())()
# OR
my_function = input()
my_function()  # Might be easier to read

# Another way of doing it:

val = raw_input()
globals()[val]()

edited Sep 12, 2013 at 9:11

answered Sep 12, 2013 at 8:55

Paco

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