A grep-like copycat to match before and after matched lines, if requested.

grep.py

#!/usr/bin/env python3
"""A `grep`-like copycat to match before and after matched lines.
"""

import re


def grep(pattern, string, before=0, after=0):
    """A grep-like copycat to match before and after matched lines,
       if requested.

    Matches a pattern in string, retrieving before and after lines.

    Args:
        pattern: a valid regular expression to match against.
        string: str to find pattern.
        before: a non negative int number of lines to retrieve
                before each matching line.
        after: a non negative int number of lines to retrieve
               after each matching line.

    Return:
        A list composed by instances of str covering
        a single match and lines before and after the match itself.

    Raises:
        TypeError: invalid parameter type.
        ValueError: invalid parameter value.
    """
    try:
        re.compile(pattern)
    except re.error:
        raise ValueError('pattern is not a valid regular expression. '
                         'Found value: {0}'
                         .format(pattern))
    if not isinstance(string, str):
        raise TypeError('string should be str. '
                        'Found type: {0}'
                        .format(type(string)))
    if not isinstance(before, int):
        raise TypeError('before should be int. '
                        'Found type: {0}'
                        .format(type(before)))
    if not isinstance(after, int):
        raise TypeError('after should be int. '
                        'Found type: {0}'
                        .format(type(after)))
    if before < 0:
        raise ValueError('before should be a non negative int. '
                         'Found value: {0}'
                         .format(before))
    if after < 0:
        raise ValueError('after should be a non negative int. '
                         'Found value: {0}'
                         .format(after))
    split_string = string.split('\n')
    return ['\n'.join(split_string[index-before:index+after+1])
            for index, value in enumerate(split_string)
            if re.findall(pattern, value)]