Helper to replace parts of a file with corresponding values

configure_file.py

import os
from string import Template


def configure_file(template_filename, out, **kwargs):
    """Rewrites placeholders within the file according to the given argument

    The file specified by template_filename is read. Occurences of template values (e.g. identifiers prefixed with '$',
    for more details see string.Template) are replaced with the corresponding keyword argument value. The resulting file
    is written to out. If out is a directory, the template's filename is stripped of the '.in' extension and used as
    the output's filename.

    @param template_filename: The filename of the template to use. It must end with '.in'. The file must be readable.
    @param out: A directory or file to write the configured file to.
    @param kwargs: Keyword arguments are used to template the file.
    """

    if not os.path.isfile(template_filename):
        raise ValueError('Template file does not exist')

    if not (os.path.splitext(template_filename)[1] == '.in' and os.path.isfile(template_filename)):
        raise ValueError('Given file is not a template file')

    if not os.access(template_filename, os.R_OK):
        raise ValueError('Cannot read template file')

    if not os.access(out, os.W_OK):
        raise ValueError('Cannot write output file')
    if os.path.isdir(out):
        output_path = os.path.join(out, os.path.splitext(os.path.basename(template_filename))[0])
    elif os.path.isfile(out):
        output_path = out
    else:
        raise ValueError('Out is neither a directory nor a file.')

    template = None
    with open(template_filename, 'r') as template_file:
        template = Template(template_file.read())

    assert(template)

    output_path = os.path.join(outdir, os.path.splitext(os.path.basename(template_filename))[0])
    with open(output_path, 'w') as output_file:
        output_file.write(template.substitute(kwargs))