If I compare two strings in python I get false even if they are the same

Question

I am trying to compare two strings, one downloaded, one from a file, but the if-statement returns always false, even if the strings are equal.

Am I doing something wrong? Is this a bug in Python?

Code:

#!/usr/bin/python
import json
import urllib2


jsonstring = urllib2.urlopen("https://xkcd.com/info.0.json").read()
j = json.loads(jsonstring)
current_xkcd = j['num']
print current_xkcd
with open ("xkcd.num", "r") as file:
        downloaded_xkcd = file.read().replace('\n', '')
print downloaded_xkcd

if current_xkcd == downloaded_xkcd:
        print "Already got latest xkcd"
else:
        print "Downloading xkcd..."

Output:

1515
1515
Downloading xkcd...

Answer 1

json.loads translates the data to Python types. You're looking at an integer and comparing it to a string.

Instead of just print current_xkcd, try print repr(current_xkcd) or print type(current_xkcd), and do the same for downloaded_xkcd.

Answer 2

Your specific problem is that you are not even comparing two strings to start with.

Json is a rich serialization protocol, and when you do json.loads with the downloaded data, whatever is numeric becomes a number of the appropriate type:

>>> import json
>>> import urllib2
>>> jsonstring = urllib2.urlopen("https://xkcd.com/info.0.json").read()
>>> j = json.loads(jsonstring)
>>> type(j["num"])
<type 'int'>

And in Python, unlike PHP, Javascript, shellscript, a string containing the "2" character does not compare equal the numeral 2. Just convert the number you've recorded in the file in an integer as well -so you are able to make the comparison:

downloaded_xkcd = int(file.read().strip())

But them, you might as well just store in the file a json representation of your data as well, or use "shelve" or "pickle" to store proper serialized data in your file, instead of doing it by hand - which leads to manual conversion strategies like your ".replace(...)" and mine "strip()" - that can (and will) fail in a lot of corner cases.