seek_and_tell_basic_example

seek_and_tell_basic_example.py

from typing import List, Optional, Any
from dataclasses import dataclass
from random import randint
from datetime import datetime, timezone
import logging

logging.basicConfig()
logger = logging.getLogger('seek_and_tell_app')
logger.setLevel(logging.DEBUG)

FILENAME: str = 'sample.txt'


def create_file(name: str, content: Optional[str] = None,) -> None:
    """Creates a file with content"""
    if not content:
        content = f'Created at {datetime.now(tz=timezone.utc)}\n'

    with open(name, 'w') as f:
        f.write(content)


@dataclass
class Reader:
    """Reader reads file and save last know position in memory"""
    filename: str
    lines: Optional[List[bytes]] = None
    last_pos: Optional[int] = None

    def update_pos(self, binary: Any) -> None:
        """Update last know position"""
        position: Any = binary.tell()
        self.last_pos = int(position)

    def read(self):
        try:
            with open(self.filename, 'rb') as f:
                if self.last_pos:
                    f.seek(self.last_pos)
                self.lines = f.readlines()
                logger.info(f"{self.lines}")
                self.update_pos(f)

        except (FileNotFoundError, OSError) as exc:
            logger.error(exc.__class__.__name__)

    def __len__(self) -> int:
        return len(self.lines)


@dataclass
class Writer:
    """Write writes N lines to given file"""
    filename: str

    def write(self, lines: int) -> None:
        try:
            with open(self.filename, 'a') as f:
                for line in range(lines):
                    f.write(f"{randint(1, 10**10)}\n")

        except (FileNotFoundError, OSError) as exc:
            logger.error(exc.__class__.__name__)


if __name__ == '__main__':
    # create a sample file
    create_file(FILENAME)

    # create Writer & Reader instances
    writer: Writer = Writer(FILENAME)
    reader: Reader = Reader(FILENAME)

    # logic
    reader.read()
    writer.write(10)     # writes 10 new lines in file
    reader.read()
    assert 10 == len(reader)
    writer.write(3)     # writes 3 new lines in sample file
    reader.read()

    # will fail, 2 != 3
    assert 2 == len(reader)