Linuxのuniqコマンドに相当するコードをPythonで実装してみました。オプションは-c、-u、-d実装しました。

.
├── uniq.py
├── test.txt

uniq.py

import sys


def uniq(f):
    previous_line = ""
    for line in f.readlines():
        if previous_line == line:
            continue

        print(line, end="")
        previous_line = line


def uniq_c(f):
    previous_line = ""
    count = 1
    for line in f.readlines():
        if previous_line == "":
            previous_line = line
            continue

        if previous_line == line:
            count += 1
            continue

        print("{} {}".format(count, previous_line), end="")
        previous_line = line
        count = 1

    print("{} {}".format(count, previous_line), end="")


def uniq_u(f):
    previous_line = ""
    is_unique = True
    for line in f.readlines():
        if previous_line == "":
            previous_line = line
            continue

        if previous_line == line:
            is_unique = False
            continue

        if is_unique:
            print(previous_line, end="")

        previous_line = line
        is_unique = True

    if is_unique:
        print(previous_line, end="")


def uniq_d(f):
    previous_line = ""
    is_repeated = False
    for line in f.readlines():
        if previous_line == "":
            previous_line = line
            continue

        if previous_line == line:
            is_repeated = True
            continue

        if is_repeated:
            print(previous_line, end="")

        previous_line = line
        is_repeated = False

    if is_repeated:
        print(previous_line, end="")


func_dictionary = {"-c": uniq_c, "-u": uniq_u, "-d": uniq_d}
try:
    if len(sys.argv) > 1:
        f = open(sys.argv[-1], "r")
        if len(sys.argv) == 2:
            uniq(f)
        elif len(sys.argv) == 3:
            if func_dictionary.get(sys.argv[1]):
                func_dictionary.get(sys.argv[1])(f)
            else:
                print("args:[-c|-u|-d]")
    else:
        f = sys.stdin
        uniq(f)
except FileNotFoundError:
    print('Error: File {0} is not found.'.format(sys.argv[1]), file=sys.stderr)
    sys.exit(1)

test.txt

aaa
bbb
aaa
aaa
AAA

以下が出力結果です。

$ python uniq.py test.txt
aaa
bbb
aaa
AAA

$ python uniq.py -c test.txt
1 aaa
1 bbb
2 aaa
1 AAA

$ python uniq.py -u test.txt
aaa
bbb
AAA

$ python uniq.py -d test.txt
aaa