(lines_enum)
Replace all environment variables that can be retrieved via `os.getenv`.

    The only allowed format for environment variables defined in the
    requirement file is `${MY_VARIABLE_1}` to ensure two things:

    1. Strings that contain a `$` aren't accidentally (partially) expanded.
    2. Ensure consistency across platforms for requirement files.

    These points are the result of a discussion on the `github pull
    request #3514 <https://github.com/pypa/pip/pull/3514>`_.

    Valid characters in variable names follow the `POSIX standard
    <http://pubs.opengroup.org/onlinepubs/9699919799/>`_ and are limited
    to uppercase letter, digits and the `_` (underscore).
Replace all environment variables that can be retrieved via `os.getenv`.
Replace all environment variables that can be retrieved via os . getenv .
def expand_env_variables(lines_enum):
    # type: (ReqFileLines) -> ReqFileLines
    """Replace all environment variables that can be retrieved via `os.getenv`.

    The only allowed format for environment variables defined in the
    requirement file is `${MY_VARIABLE_1}` to ensure two things:

    1. Strings that contain a `$` aren't accidentally (partially) expanded.
    2. Ensure consistency across platforms for requirement files.

    These points are the result of a discussion on the `github pull
    request #3514 <https://github.com/pypa/pip/pull/3514>`_.

    Valid characters in variable names follow the `POSIX standard
    <http://pubs.opengroup.org/onlinepubs/9699919799/>`_ and are limited
    to uppercase letter, digits and the `_` (underscore).
    """
    for line_number, line in lines_enum:
        for env_var, var_name in ENV_VAR_RE.findall(line):
            value = os.getenv(var_name)
            if not value:
                continue

            line = line.replace(env_var, value)

        yield line_number, line
def expand_env_variables ( lines_enum ) : # type: (ReqFileLines) -> ReqFileLines for line_number , line in lines_enum : for env_var , var_name in ENV_VAR_RE . findall ( line ) : value = os . getenv ( var_name ) if not value : continue line = line . replace ( env_var , value ) yield line_number , line