PHP - Function name must be a string

The error:

PHP Notice:  Undefined variable: exec in readings.php on line 3
PHP Fatal error:  Function name must be a string in readings.php on line 3

The code:

<?php
    require('smarty_config.php');
    exec('reading_fetcher.py',$output,$ret_code);
    $smarty->assign('readings',$output);
    $smarty->display('readings.tpl');
?>

I was asked for the code of reading_fetcher.py so here it is:

#!/usr/bin/env python
import urllib2, re

response = urllib2.urlopen('http://it.ctsfw.edu/inc/nc_scriptureframe.php')

html = response.read()

def remove_html_tags(data):
    p = re.compile(r'<.*?>')
    return p.sub(' ', data)

import re
import htmlentitydefs

def convertentity(m):
    if m.group(1)=='#':
        try:
            return unichr(int(m.group(2)))
        except ValueError:
            return '&#%s;' % m.group(2)
        try:
            return htmlentitydefs.entitydefs[m.group(2)]
        except KeyError:
            return '&%s;' % m.group(2)

def converthtml(s):
    return re.sub(r'&(#?)(.+?);',convertentity,s)

readings =  converthtml(str(remove_html_tags(html)))
readings.replace("&nbsp;", " ")

print readings[699:]

I already looked here, here and here. Two of those errors are an extra "$". I don't see an extra "$" on my function name. The third error is having "()" instead of "[]". So I tried replacing them. That didn't work. What else might I try?