Creating a set from a variable instead of a file

Question

I have a piece of code to read two files, convert them to sets, and then subtract one set from the other. I would like to use a string variable (installedPackages) for "a" instead of a file. I would also like to write to a variable for "c".

a = open("/home/user/packages1.txt")
b = open("/home/user/packages.txt")
c = open("/home/user/unique.txt", "w")

for line in set(a) - set(b):
    c.write(line)

a.close()
b.close()
c.close()

I have tried the following and it does not work:

for line in set(installedPackages) - set(b):

I have tried to use StringIO, but I think I am using it improperly.

Here, finally, is how I have created installedPackages:

stdout, stderr = p.communicate()
installedPackages = re.sub('\n$', '', re.sub('install$', '', re.sub('\t', '', stdout), 0,re.MULTILINE))

Sample of packages.txt:

humanity-icon-theme
hunspell-en-us
hwdata
hyphen-en-us
ibus
ibus-gtk
ibus-gtk3
ibus-pinyin
ibus-pinyin-db-android
ibus-table

You... you've just gone and scrapped the newline characters. What are you wanting to make your set from? Lines? Words? — Chris Morgan
– Chris Morgan, Commented May 15, 2012 at 23:44
One word on each line.Each line should be a new entry in set. — Poweruser32
– Poweruser32, Commented May 15, 2012 at 23:48
Sorry, actually re.sub('\n$', '', ...) will not change the input. So that just leaves it as dead (and misleading) code. — Chris Morgan
– Chris Morgan, Commented May 15, 2012 at 23:51

jamylak · Accepted Answer · 2012-05-15 23:29:50Z

2

If you want to write to a string buffer file-like use StringIO

>>> from StringIO import StringIO
>>> installed_packages = StringIO()
>>> installed_packages.write('test')
>>> installed_packages.getvalue()
'test'

answered May 15, 2012 at 23:29

jamylak

134k30 gold badges238 silver badges240 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Poweruser32 Over a year ago

installed_packages.getvalue() produces what I want, but "print set(installed_packages.getvalue())" returns set(['\n', '+', '-', '.', '1', '0', '3', '2', '5', '4', '7', '6', '9', '8', ':', 'a', 'c', 'b', 'e', 'd', 'g', 'f', 'i', 'h', 'k', 'j', 'm', 'l', 'o', 'n', 'q', 'p', 's', 'r', 'u', 't', 'w', 'v', 'y', 'x', 'z'])

Chris Morgan Over a year ago

If the user controls the code, StringIO is often not appropriate. Its main useful spot is where there is code which is expecting a file object and can't be changed easily. It sounds in this case like the @Poweruser32 is just wanting to switch from files to strings.

Hugh Bothwell · Accepted Answer · 2012-05-16 01:06:48Z

1

Something like the following?

Edit: after several iterations:

from subprocess import Popen, PIPE

DEBUG = True
if DEBUG:
    def log(msg, data):
        print(msg)
        print(repr(data))
else:
    def log(msg, data):
        pass

def setFromFile(fname):
    with open(fname) as inf:
        return set(ln.strip() for ln in inf)

def setFromString(s):
    return set(ln.strip() for ln in s.split("\n"))

def main():
    # get list of installed packages
    p = Popen(['dpkg', '--get-selections'], stdout=PIPE, stderr=PIPE)
    stdout, stderr = p.communicate()
    installed_packages = setFromString(stdout)

    # get list of expected packages
    known_packages = setFromFile('/home/john/packages.txt')

    # calculate the difference
    unknown_packages = installed_packages - known_packages
    unknown_packages_string = "\n".join(unknown_packages)

    log("Installed packages:", installed_packages)
    log("Known packages:", known_packages)
    log("Unknown packages:", unknown_packages)

if __name__=="__main__":
    main()

edited May 16, 2012 at 1:06

answered May 16, 2012 at 0:05

Hugh Bothwell

57k9 gold badges91 silver badges103 bronze badges

6 Comments

Poweruser32 Over a year ago

Sorry I just started to learn python today. I do not understand why I would get an indentation error.

File "test.py", line 16     installed_packages = setFromString(stdout)                                              ^ IndentationError: unindent does not match any outer indentation level

Poweruser32 Over a year ago

I unindented the main() function, but now it is saying stdout is not defined. installed_packages = setFromString(stdout) NameError: name 'stdout' is not defined

Hugh Bothwell Over a year ago

@Poweruser32: I've included the subprocess creation, if you set the name of your script properly it ought to work.

Poweruser32 Over a year ago

I tried this. pastebin.com/TnStVk5w and it just gives me the raw output of the dpkg command i give it.

Hugh Bothwell Over a year ago

@Poweruser32: try printing installed_packages and known_packages, make sure they are populating correctly? If known_packages is empty, unknown_packages will be identical to installed_packages (which sounds like what you are describing).

|

garnertb · Accepted Answer · 2012-05-15 23:31:05Z

1

The set data type takes an iterable as a parameter, therefore if installedPackages a string with multiple items you need to split it by the delimiter. For example, the following code would split the string by all commas:

for line in set(installedPackages.split(',')) - set(b):
    c.write(line)

answered May 15, 2012 at 23:31

garnertb

9,62439 silver badges38 bronze badges

2 Comments

Poweruser32 Over a year ago

I just tried "for line in set(installedPackages.split('\n')) - set(b):" and it produces no output

garnertb Over a year ago

Edit your question to show the code where you assign the string to installedPackages.

Collectives™ on Stack Overflow

Creating a set from a variable instead of a file

3 Answers 3

2 Comments

6 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

6 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related