当从python调用时，Grep不断返回0

Question

Background: I have a lot of music. I also tend to listen to the same albums over and over. Eventually, this annoyed me to the point where I decided I'd listen to a different one each day. Seeing as I have just over 100 albums in my collection, and seeing as I know a little programming, I thought I'd write a script to help myself out.

Now, keep in mind, I'm a student who knows he knows very little. Most of what I know has been from a single Python book (though a decent one for getting beginners off the ground) and the Python documentation.

Also, I'm running Python 3.6 and using Cygwin, so I do have *nix commands available.

The problem lies in the following method (edit 9:43 EST - suggested "if line in file" implementation added; new lines starred):

def choose_album(library_file, day_file):
    albums = []
    for line in fileinput.input(library_file):
        albums.append(line)
    fileinput.close() #**
    check_file = io.open(day_file) #**
    album_chosen = False #**
    while album_chosen == False:
        choice = random.randrange(0, len(albums))
        print("Searching for " + albums[choice] + ".")
        for line in check_file: #**
            if albums[choice] in line: #**
                break #**
        if albums[choice] not in check_file: #**
            album_chosen = True #**
    print(albums[choice])
    return albums[choice]

where the library_file is a text file with all of the albums I have purchased or listened to on Spotify and the day_file is my "album of the day" file, each line containing a date and the album for that day. The while loop used to be an

if albums[choice] in line:
    break

and then the script would go on to write that to the day_file. It worked fine, but the problem was that the if albums[choice] in line line would sometimes return false negatives, giving me an album I'd already listened to, meaning I'd have to go back into my file, delete the redundant album, and run the script again.

Update (9:43 EST) Based on my test run of the edited code, this at least doesn't spit out heaps of text. Is it likely to give me duplicates again?

Update (12:02 EST) - removed output that became irrelevant due to suggestion.

Answer 1

You are reading lines without trimming the final newline:

for line in fileinput.input(library_file):
    albums.append(line)  ##### BUG

What you probably meant to write might be along the lines of

    albums.append(line.rstrip('\n'))

The newline caused grep to print out every line. Somewhat to my surprise, after 25+ years as an active Unix user, I discovered that at least some versions of grep will cause

 grep 'String which ends with newline
 ' filename

to print out every line in filename , apparently because it considers this equivalent to

grep -e 'String which ends with newline' -e '' filename`

where obviously the empty string as a regular expression matches every possible input.

Switching from grep to if albums[choice] in line removes this symptom, and actually happens to work because you are also not trimming newlines from lines you read from the other file, and you specifically look for matches which are at the end of the line; and so "foo\\n" in "datestamp foo\\n" actually returns True . However, this appears to be contrary to your expectations (for example, you are printing the string followed by a full stop, which lands after the embedded newline, on a line by itself in the output) so perhaps this is still a (minor) bug in your code.