Python dir() function with variable arguments

Question

For fun/to practice python, I am trying to create a program that displays the contents of a module. Looks like this:

import sys
print "Here are all the functions in the sys module: "
print dir(sys)
function = raw_input("Pick a function to see: ")

cnt = True

while cnt:
        if function in dir(sys):
                print "You chose:", function
                cnt = False
        else:
                print "Invalid choice, please select again."
                cnt = True
print dir("sys." + function)

But every time, no matter what string the variable function is set to, the dir("sys." + function) call always defaults to the same output as dir(string) (or so I think!)

What is happening and is there a way for me to do this properly and get the output I really want (for example, the variable function is set to stdin and I get the output for dir(sys.stdin))?

use while cnt: instead of while cnt == True:

jfs
– jfs

2013-02-12 14:44:54 +00:00
Commented Feb 12, 2013 at 14:44 — jfs
– jfs, Commented Feb 12, 2013 at 14:44

Martijn Pieters · Accepted Answer · 2013-02-12 14:41:36Z

4

You want to retrieve the actual object from the module; use the getattr() function for that:

print dir(getattr(sys, function))

dir() does not interpret the contents of the objects you pass to it; a string that happens to contain a value that corresponds to the name of a function in a module is not dereferenced for you.

answered Feb 12, 2013 at 14:41

Martijn Pieters

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7 Comments

eyquem Over a year ago

@Pieters What senses have the words string and value in your sentence please ?

Martijn Pieters Over a year ago

@eyquem: String: the str type in python. Value: the contents (characters) of the string.

eyquem Over a year ago

@Pieters You mean, for string: an object of type 'str' ?

eyquem Over a year ago

@eyquem So you meant: "an object of type 'str' that happens to contain a value that corresponds..." , am I right ?

Martijn Pieters Over a year ago

Yes. Where is this going? We usually talk about strings, not about "an object of type str", because that's cumbersome and overly verbose.

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jfs · Accepted Answer · 2013-02-12 14:44:03Z

2

f = getattr(sys, function) # get function object
help(f) # display help for the function

Note: dir(f) would return the same info for all functions with the same type.

answered Feb 12, 2013 at 14:44

jfs

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Comments

Michal Čihař · Accepted Answer · 2013-02-12 14:43:29Z

1

As dir works on objects, you need to get the object somehow from the name.

First option is to use getattr:

print dir(getattr(sys, function))

If you want to be more flexible (and are ready to take security risk), you can use eval:

print dir(eval('sys.' + function))

answered Feb 12, 2013 at 14:43

Michal Čihař

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