How to slice strings backwards python
Question: I am learning python and I made a basic program where the user takes the link
of a photo and inputs it, then the program downloads that photo. In order to
make sure that the user doesn't enter a link that is a webpage instead of a
photo, I had the program check what the file extension was by using string
slicing, but I can't seem to find out how to slice the string backwards

I know that this is an dumb question but after an hour of searching I still
can't find the answer. Here is the code

    
    
    import random
    import urllib.request
    import urllib.parse
    
    def download_web_image(url, file_format):
        try:
            name = random.randrange(1, 1000)
            full_name = str(name) + file_format
            urllib.request.urlretrieve(url, full_name)
            print("Image download successful!")
            print("Image named " + full_name)
        except:
            print('Error')
    
    
    def get_user_url():
        url = input("Now enter the url of the photo you want to download:")
        try:
    
            if url[0:3:-1] is '.png':
                download_web_image(url, ".png")
            elif url[0:4:-1] is 'gepj.':
                download_web_image(url, '.jpeg')
            elif url[0:3:-1] is '.gpj':
                download_web_image(url, '.jpg')
            else:
                print('the file format is uncompatible: ' + url[1:4:-1])
    
        except:
            print('The url is not valid!')
    
    print('look for an image on a website, make sure it is a JPG or PNG file or it will not work!')
    get_user_url()
    

Thank you for the help. and no, I do not want the string to show up backwards.


Answer: I suggest you use the built-in method
[`endswith`](https://docs.python.org/2.7/library/stdtypes.html?highlight=endswith#str.endswith)
saves the trouble of variable size extensions(`png`,`jpeg`,`jpg`...etc), this
way:

    
    
    >>>url = 'https://www.python.org/static/community_logos/python-logo-master-v3-TM.png'
    >>>url.endswith('.png')
    True
    

