Weather powered by Python

No, I haven't gone delusional in my ever-continuing worship of Python and exaggerated its capabilities. My friend and fellow hacker Thomas Upton posted a clever script to obtain weather forecasts from Yahoo! RSS feeds, specifically for the use in Geek Tool.

I wanted to point out to Thomas a couple of areas of improvement on his blog, but unfortunately, he doesn't seem to have commenting enabled on his Wordpress blog because he's paranoid that his site will get pwned by pr0n ads. (Rightly so, it's PHP software after all.) Hence, I'll post my response here.

I made a few revisions to Thomas's code. Namely, I ported it from the getopt Python standard library module, which is somewhat deprecated, to another handy stdlib module, optparse. I also removed the option for the two-day forecast and replaced it with an argument for the number of days to forecast. I may have changed an option name or two, as well.

You can see the expressiveness of optparse compared to getopt. The boilerplate is standard for optparse and prevents you from having to roll your own for a lot, including help documentation, usage statements, and most helpfully, in parsing the CLI for errors.

The final thing of note is the movement to string formatting as opposed to lots of string concatenation. String formatting is considered more Pythonic. Of course string formatting is going to be a lot easier and prettier in Python 3.0; thus this script will definitely have to get overhauled for that (though the 2to3 tool should take care of this). You'd have to do it anyways, though, because print will no longer be a statement but a function in Python 3.0.

Without further ado, you can have a look at my modifications at http://rafb.net/p/YWiyo548.html.

You can also apply a patch if you would like, which I supply here:

=== modified file 'weather.py'
--- weather.py  2008-04-23 20:11:38 +0000
+++ weather.py  2008-04-23 21:48:58 +0000
@@ -1,45 +1,67 @@
-#! /usr/bin/python
+#!/usr/bin/env python
+
+"""Fetches weather reports from Yahoo!"""

import sys
import urllib
-import getopt
+from optparse import OptionParser
from xml.dom.minidom import parse

+# Yahoo!'s limit on the number of days they will forecast for
+DAYS_LIMIT = 2
WEATHER_URL = 'http://xml.weather.yahoo.com/forecastrss?p=%s'
WEATHER_NS = 'http://xml.weather.yahoo.com/ns/rss/1.0'

-def get_weather(zip_code):
+def get_weather(zip_code, days):
+    """
+    Fetches weather report from Yahoo!
+
+    :Parameters:
+    -`zip_code`: A five digit US zip code.
+    -`days`: number of days to obtain forecasts for
+
+    :Returns:
+    -`weather_data`: a dictionary of weather data
+
+    """
+
# Get the correct weather url.
url = WEATHER_URL % zip_code

# Parse the XML feed.
dom = parse(urllib.urlopen(url))
-
+
# Get the units of the current feed.
yunits = dom.getElementsByTagNameNS(WEATHER_NS, 'units')[0]
-
+
# Get the location of the specified zip code.
ylocation = dom.getElementsByTagNameNS(WEATHER_NS, 'location')[0]
-
+
# Get the currrent conditions.
ycondition = dom.getElementsByTagNameNS(WEATHER_NS, 'condition')[0]

# Hold the forecast in a hash.
forecasts = []
-
+
# Walk the DOM in order to find the forecast nodes.
-    for node in dom.getElementsByTagNameNS(WEATHER_NS,'forecast'):
-  
-        # Insert the forecast into the forcast hash.
-        forecasts.append ({
-            'date': node.getAttribute('date'),
-            'low': node.getAttribute('low'),
-            'high': node.getAttribute('high'),
-            'condition': node.getAttribute('text')
-        })
-
-    # Return a hash of the weather that we just parsed.
-    return {
+    for i, node in enumerate(
+            dom.getElementsByTagNameNS(WEATHER_NS,'forecast')):
+        # stop if obtained forecasts for number of requested days
+        if i + 1 > days:
+            break
+        else:
+            # Insert the forecast into the forcast dictionary.
+            forecasts.append (
+                {
+                    'date': node.getAttribute('date'),
+                    'low': node.getAttribute('low'),
+                    'high': node.getAttribute('high'),
+                    'condition': node.getAttribute('text')
+                }
+            )
+
+    # Return a dictionary of the weather that we just parsed.
+    weather_data = {
   'current_condition': ycondition.getAttribute('text'),
   'current_temp': ycondition.getAttribute('temp'),
   'forecasts': forecasts,
@@ -47,87 +69,117 @@
   'city': ylocation.getAttribute('city'),
   'region': ylocation.getAttribute('region'),
}
-
-def usage():
-    print "Usage: weather.py [-cflv] zip-code\n"
-    print "-c\tSuppress the current weather.\n"
-    print "-f\tPrint the next two days' forecast.\n"
-    print "-l\tPrint the location of the weather.\n"
-    print "-v\tPrint headers for each weather section.\n"
-
-def main():
-    try:
-        # Attempt to get the command line arguments and options.
-        opts, args = getopt.getopt(sys.argv[1:], "cflv")
-
-    except GetoptError, err:
-
-        # Print the getopt() error.
-        print str(err)
-  
-        # Show the user the proper usage.
-        usage()
-  
-        # Exit with error code 2.
-        sys.exit(2)
-
+    return weather_data
+
+
+def create_report(weather_data, options):
+    """
+    Constructs a weather report as a string.
+
+    :Parameters:
+    -`weather_data`: a dictionary of weather data
+    -`options`: options to determine output selections
+
+    :Returns:
+    -`report_str`: a formatted string reporting weather
+
+    """
+
+    report = []
+    if options.location:
+        if options.verbose:
+            # Add location header
+            report.append("Location:")
+
+        # Add the location
+        locstr = "%(city)s, %(region)s\n" % weather_data
+        report.append(locstr)
+
+    if (not options.nocurr):
+        if options.verbose:
+            # Add current conditions header
+            report.append("Current conditions:")
+
+        # Add the current weather.
+        currstr = "%(current_temp)s%(units)s | %(current_condition)s \n" % weather_data
+        report.append(currstr)
+
+    if options.verbose:
+        # Add the forecast header.
+        report.append("Forecast:")
+
+    # Add the forecasts.
+    for forecast in weather_data['forecasts']:
+        forecast['units'] = weather_data['units']
+        forecast_str = """\
+%(date)s
+  Low: %(low)s%(units)s
+  High: %(high)s%(units)s
+  Condition: %(condition)s
+""" % forecast
+        report.append(forecast_str)
+
+    report_str = "\n".join(report)
+    return report_str
+
+
+def create_cli_parser():
+    """Creates command line interface parser."""
+
+    usage = (
+        "python %prog [OPTIONS] ZIPCODE DAYS",
+        __doc__,
+        """\
+Arguments:
+    ZIPCODE: The ZIP code for the region of interest.
+    DAYS: Days to forecast for (0 if only current conditions desired).
+"""
+    )
+    usage = "\n\n".join(usage)
+    cli_parser = OptionParser(usage)
+    # add CLI options
+    cli_parser.add_option('-c', '--nocurr', action='store_true',
+        help="Suppress reporting the current weather conditions"
+    )
+    cli_parser.add_option('-l', '--location', action='store_true',
+        help="Give the location of the weather"
+    )
+    cli_parser.add_option('-v', '--verbose', action='store_true',
+        help="Print the weather section headers"
+    )
+
+    return cli_parser
+
+
+def main(argv):
+
+    cli_parser = create_cli_parser()
+    opts, args = cli_parser.parse_args(argv)
+
# Check that an argument was passed.
-    if not args:
-  
-        # Show the user the proper usage.
-        usage()
-  
-        # Exit with error code 1.
-        sys.exit(1)
-
-    # Check that the first argument exists.
-    if args[0]:
-
-        # Get the weather.
-        weather = get_weather(args[0])
-
-        # Set the option flags
-        c = True
-        f = False
-        l = False
-        v = False
-
-        # Check the options
-        for o, a in opts:
-            if o == "-c":
-                c = False
-            elif o == "-f":
-                f = True
-            elif o == "-l":
-                l = True
-            elif o == "-v":
-                v = True
-      
-        if l and v:
-            # Print the location header.
-            print "Location:"
-
-        if l:
-            # Print the location
-            print weather['city'] + ", " + weather['region'] + "\n"
-  
-        if c and v:
-            # Print the current conditions header.
-            print "Current conditions:"
-
-        if c:
-            # Print the current weather.
-            print weather['current_temp'] + weather['units'] + " | " + weather['current_condition'] + "\n"
-
-        if f and v:
-            # Print the forecast header.
-            print "Forecast:"
-
-        if f:
-            # Print the forecast.
-            for forecast in weather['forecasts']:
-                print forecast['date'] + "\n  Low: " + forecast['low'] + weather['units'] + "\n  High: " + forecast['high'] + weather['units'] + "\n  Condition: " + forecast['condition'] + "\n"
-          
+    if len(args) != 2:
+        cli_parser.error("Not enough arguments supplied.")
+
+    zip_code = args[0]
+    if len(zip_code) != 5 or not zip_code.isdigit():
+        cli_parser.error("ZIP code must be 5 digits")
+
+    days = args[1]
+    if not days.isdigit():
+        cli_parser.error("Days to forecast must be an integer.")
+    days = int(days)
+    if days <> DAYS_LIMIT:
+        cli_parser.error("Days to forecast must be within 0 to %d" %
+                DAYS_LIMIT)
+
+    # Get the weather forecast.
+    weather = get_weather(zip_code, days)
+
+    # Create the report.
+    report = create_report(weather, opts)
+
+    print report
+
+
if __name__ == "__main__":
-    main()
-
\ No newline at end of file
+    main(sys.argv[1:])

I'm going to hope Thomas doesn't sue me for breaking copyright or something, and suck money out of my massive salary that I receive as a grad student.

EDIT: One final comment. Thomas, your code looked very clean and you did a fantastic job adhering to PEP 8!