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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def pkg_upgrade(repo, skip, flag):
 """Checking packages for upgrade """

    Msg().checking()
    PACKAGES_TXT = RepoInit(repo).fetch()[0]
    pkgs_for_upgrade = []
    # name = data[0]
    # location = data[1]
    # size = data[2]
    # unsize = data[3]
    data = repo_data(PACKAGES_TXT, repo, flag="")
    for pkg in installed():
        status(0.0005)
        inst_pkg = split_package(pkg)
        for name in data[0]:
            if name:    # this tips because some pkg_name is empty
                repo_pkg = split_package(name[:-4])
            if (repo_pkg[0] == inst_pkg[0] and
                LooseVersion(repo_pkg[1]) > LooseVersion(inst_pkg[1]) and
                repo_pkg[3] >= inst_pkg[3] and
                    inst_pkg[0] not in skip and
                    repo_pkg[1] != "blacklist"):
                pkgs_for_upgrade.append(repo_pkg[0])
    Msg().done()
    if "--checklist" in flag:
        pkgs_for_upgrade = choose_upg(pkgs_for_upgrade)
    return pkgs_for_upgrade