在 Python 上按关键字对 txt 中的行进行排序

Question

I faced with problem on python: I want to sort.txt like in desired output below But instead of this output I get wrong output with concatenated first and second lines and blank line at the end of file Why is this happening?

Thanks in advance for any help

Input file:

https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/
https://markus.rmart.ru/engine/preloader/

Desired output:

https://markus.rmart.ru/engine/preloader/
https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

Real output:

https://markus.rmart.ru/engine/preloader/https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

Code:

test_out = open('./test_out999.txt', "w")

def my_sort(line):
    social_folders = {'engine': 1,
                    'wormix_mm': 2,
                    'wormix_ok': 3}
    line_fields = line.strip().split("/")
    social = line_fields[3]
    print(line_fields[3])
    return social_folders[social]

testsortf = open('./testsort.txt')
contents = testsortf.readlines()

contents.sort(key=my_sort)

for line in contents:
        test_out.write(line)

testsortf.close()
test_out.close()

But when I delete last "\n" with line.rstrip('\n') and add "\n" manually, I take this output (with unwanted blank line at the end of file):

https://markus.rmart.ru/engine/preloader/
https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

Small fix:

test_out.write(line.rstrip('\n') + "\n")

So, why did it happen and how me to get desired output?

And, if anyone can help me with problem, next... How to get this output?

First:
https://markus.rmart.ru/engine/preloader/

Second:
https://markus.rmart.ru/wormix_mm/preloader/

Third:
https://markus.rmart.ru/wormix_ok/preloader/

Answer 1

When you add \n to each line, the \n is added to the last line as well. On every line apart from the newline, something is written on the newline created before - however, on the last line, nothing is written in that newline, leaving it blank. Here is an example:

Iteration 1:

https://markus.rmart.ru/wormix_mm/preloader/

Iteration 2:

https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

Notice how that newline that we created in Interation 1 has text in it now. If there was no newline, it would look like this:

https://markus.rmart.ru/wormix_mm/preloader/https://markus.rmart.ru/wormix_ok/preloader/

because the text is written from the end of the file.

Finally, Iteration 3:

https://markus.rmart.ru/engine/preloader/
https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

As you can see, nothing is being written after Iteration 3, leaving the last line blank.

In order to fix that, you would have to do a simple check to see if the line is currently the last line (replace your for line in contents for loop with this):

for i in range(len(contents)):
    test_out.write(line.rstrip('\n'))
    if i < len(contents) - 1:
        test_out.write("\n")

In order to do what you want (the First , Second , Third ), you could just have a list full of those words:

num_to_word = ["First", "Second", "Third"]
for i in range(len(contents)):
    test_out.write(num_to_word[i] + ":\n")
    test_out.write(line.rstrip('\n'))
    if i < len(contents) - 1:
        test_out.write("\n\n") # Two newlines to add a line in between

(I haven't tested this, please tell me if it doesn't work)

Answer 2

Your unexpected:

https://markus.rmart.ru/engine/preloader/https://markus.rmart.ru/wormix_mm/preloader/
https://markus.rmart.ru/wormix_ok/preloader/

is because the last line of the input file doesn't have the newline.
So if we mark newlines as △ :

Input file:

 https://markus.rmart.ru/wormix_mm/preloader/△ https://markus.rmart.ru/wormix_ok/preloader/△ https://markus.rmart.ru/engine/preloader/

So 2 elements of content have \n suffixed, and 1 doesn't, causing the different behavior.
The simple fix would be not adding an extra newline every time , but only to the last:

contents = testsortf.readlines()
contents[-1] = f'{contents[-1]}\n'

If contents may be empty:

contents = testsortf.readlines()
if contents:
    contents[-1] = f'{contents[-1]}\n'

---

So we now have the code:

test_out = open('...', "w")

def my_sort(line):
    social_folders = {'engine': 1,
                    'wormix_mm': 2,
                    'wormix_ok': 3}
    line_fields = line.strip().split("/")
    social = line_fields[3]
    print(line_fields[3])
    return social_folders[social]

testsortf = open('...')
contents = testsortf.readlines()
contents[-1] = f'{contents[-1]}\n'
contents.sort(key=my_sort)
for line in contents:
    test_out.write(line)

testsortf.close()
test_out.close()

In order to add First , Second etc., first add a tuple , eg

numbers = 'First', 'Second', 'Third'

And then use the handy enumerate() :

test_out = open('./test_out999.txt', "w")

def my_sort(line):
    social_folders = {'engine': 1,
                    'wormix_mm': 2,
                    'wormix_ok': 3}
    line_fields = line.strip().split("/")
    social = line_fields[3]
    print(line_fields[3])
    return social_folders[social]

numbers = 'First', 'Second', 'Third'  # <---
testsortf = open('./testsort.txt')
contents = testsortf.readlines()
contents[-1] = f'{contents[-1]}\n'
contents.sort(key=my_sort)
for i, line in enumerate(contents):
    test_out.write(f'{numbers[i]}:\n{line}')  # No., newline, content
    if i+1 < len(contents):  # Don't add additional \n for last line
        test_out.write('\n')

testsortf.close()
test_out.close()

---

An additional suggestion:
Using with... as f is good practice in Python, because it closes the file even if errors occur. So the final code:

def my_sort(line):
    social_folders = {'engine': 1,
                    'wormix_mm': 2,
                    'wormix_ok': 3}
    line_fields = line.strip().split("/")
    social = line_fields[3]
    print(line_fields[3])
    return social_folders[social]

numbers = 'First', 'Second', 'Third', 'Fourth'
with open('./testsort.txt') as testsortf, \
     open('./test_out999.txt', "w") as test_out:
    contents = testsortf.readlines()
    contents[-1] = f'{contents[-1]}\n'
    contents.sort(key=my_sort)
    for i, line in enumerate(contents):
        test_out.write(f'{numbers[i]}:\n{line}')
        if i+1 < len(contents):  # Don't add additional \n for last line
            test_out.write('\n')
    # No need to call close()!

Notes

1. See PEP 279 for more information about enumerate() .
2. f-strings ( f'...{...}...' ) were added in Python 3.6 by PEP 498 . Use '...{}...'.format(...) for Python 3.5 or lower.