如何向后切字符串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 1

I suggest you use the built-in method 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

Answer 2

What you want is to index by negative numbers, which search from the end of the list/string:

extension = url[-3:] will retrieve the last 3 characters in url .

url[-1] is the last character of url , url[-2] is the 2nd-to-last, and so on. So url[-3:] gets everything from the 3rd-to-last character to the end of the string.

Answer 3

The following type of approach might be easier to follow:

def get_user_url():
    url = input("Now enter the url of the photo you want to download: ")

    try:
        extension = os.path.splitext(url.lower())[1]

        if extension in [".png", ".jpeg", ".jpg"]:
            download_web_image(url, extension)
        else:
            print('the file format is uncompatible: {:}'.format(extension))

    except:
        print('The url is not valid!')

I suggest you also convert your input into lowercase so that you can catch ".JPG" at the same time.