scrapy/scrapy/utils/misc.py

"""Helper functions which don't fit anywhere else"""
import ast
import hashlib
import inspect
import os
import re
import warnings
from collections import deque
from contextlib import contextmanager
from functools import partial
from importlib import import_module
from pkgutil import iter_modules
from types import ModuleType
from typing import (
    IO,
    TYPE_CHECKING,
    Any,
    Callable,
    Deque,
    Generator,
    Iterable,
    List,
    Optional,
    Pattern,
    Union,
    cast,
)

from w3lib.html import replace_entities

from scrapy.item import Item
from scrapy.utils.datatypes import LocalWeakReferencedCache
from scrapy.utils.deprecate import ScrapyDeprecationWarning
from scrapy.utils.python import flatten, to_unicode

if TYPE_CHECKING:
    from scrapy import Spider


_ITERABLE_SINGLE_VALUES = dict, Item, str, bytes


def arg_to_iter(arg: Any) -> Iterable[Any]:
    """Convert an argument to an iterable. The argument can be a None, single
    value, or an iterable.

    Exception: if arg is a dict, [arg] will be returned
    """
    if arg is None:
        return []
    if not isinstance(arg, _ITERABLE_SINGLE_VALUES) and hasattr(arg, "__iter__"):
        return cast(Iterable[Any], arg)
    return [arg]


def load_object(path: Union[str, Callable]) -> Any:
    """Load an object given its absolute object path, and return it.

    The object can be the import path of a class, function, variable or an
    instance, e.g. 'scrapy.downloadermiddlewares.redirect.RedirectMiddleware'.

    If ``path`` is not a string, but is a callable object, such as a class or
    a function, then return it as is.
    """

    if not isinstance(path, str):
        if callable(path):
            return path
        raise TypeError(
            f"Unexpected argument type, expected string or object, got: {type(path)}"
        )

    try:
        dot = path.rindex(".")
    except ValueError:
        raise ValueError(f"Error loading object '{path}': not a full path")

    module, name = path[:dot], path[dot + 1 :]
    mod = import_module(module)

    try:
        obj = getattr(mod, name)
    except AttributeError:
        raise NameError(f"Module '{module}' doesn't define any object named '{name}'")

    return obj


def walk_modules(path: str) -> List[ModuleType]:
    """Loads a module and all its submodules from the given module path and
    returns them. If *any* module throws an exception while importing, that
    exception is thrown back.

    For example: walk_modules('scrapy.utils')
    """

    mods: List[ModuleType] = []
    mod = import_module(path)
    mods.append(mod)
    if hasattr(mod, "__path__"):
        for _, subpath, ispkg in iter_modules(mod.__path__):
            fullpath = path + "." + subpath
            if ispkg:
                mods += walk_modules(fullpath)
            else:
                submod = import_module(fullpath)
                mods.append(submod)
    return mods


def extract_regex(
    regex: Union[str, Pattern], text: str, encoding: str = "utf-8"
) -> List[str]:
    """Extract a list of unicode strings from the given text/encoding using the following policies:

    * if the regex contains a named group called "extract" that will be returned
    * if the regex contains multiple numbered groups, all those will be returned (flattened)
    * if the regex doesn't contain any group the entire regex matching is returned
    """
    warnings.warn(
        "scrapy.utils.misc.extract_regex has moved to parsel.utils.extract_regex.",
        ScrapyDeprecationWarning,
        stacklevel=2,
    )

    if isinstance(regex, str):
        regex = re.compile(regex, re.UNICODE)

    try:
        # named group
        strings = [regex.search(text).group("extract")]  # type: ignore[union-attr]
    except Exception:
        # full regex or numbered groups
        strings = regex.findall(text)
    strings = flatten(strings)

    if isinstance(text, str):
        return [replace_entities(s, keep=["lt", "amp"]) for s in strings]
    return [
        replace_entities(to_unicode(s, encoding), keep=["lt", "amp"]) for s in strings
    ]


def md5sum(file: IO) -> str:
    """Calculate the md5 checksum of a file-like object without reading its
    whole content in memory.

    >>> from io import BytesIO
    >>> md5sum(BytesIO(b'file content to hash'))
    '784406af91dd5a54fbb9c84c2236595a'
    """
    m = hashlib.md5()
    while True:
        d = file.read(8096)
        if not d:
            break
        m.update(d)
    return m.hexdigest()


def rel_has_nofollow(rel: Optional[str]) -> bool:
    """Return True if link rel attribute has nofollow type"""
    return rel is not None and "nofollow" in rel.replace(",", " ").split()


def create_instance(objcls, settings, crawler, *args, **kwargs):
    """Construct a class instance using its ``from_crawler`` or
    ``from_settings`` constructors, if available.

    At least one of ``settings`` and ``crawler`` needs to be different from
    ``None``. If ``settings `` is ``None``, ``crawler.settings`` will be used.
    If ``crawler`` is ``None``, only the ``from_settings`` constructor will be
    tried.

    ``*args`` and ``**kwargs`` are forwarded to the constructors.

    Raises ``ValueError`` if both ``settings`` and ``crawler`` are ``None``.

    .. versionchanged:: 2.2
       Raises ``TypeError`` if the resulting instance is ``None`` (e.g. if an
       extension has not been implemented correctly).
    """
    if settings is None:
        if crawler is None:
            raise ValueError("Specify at least one of settings and crawler.")
        settings = crawler.settings
    if crawler and hasattr(objcls, "from_crawler"):
        instance = objcls.from_crawler(crawler, *args, **kwargs)
        method_name = "from_crawler"
    elif hasattr(objcls, "from_settings"):
        instance = objcls.from_settings(settings, *args, **kwargs)
        method_name = "from_settings"
    else:
        instance = objcls(*args, **kwargs)
        method_name = "__new__"
    if instance is None:
        raise TypeError(f"{objcls.__qualname__}.{method_name} returned None")
    return instance


@contextmanager
def set_environ(**kwargs: str) -> Generator[None, Any, None]:
    """Temporarily set environment variables inside the context manager and
    fully restore previous environment afterwards
    """

    original_env = {k: os.environ.get(k) for k in kwargs}
    os.environ.update(kwargs)
    try:
        yield
    finally:
        for k, v in original_env.items():
            if v is None:
                del os.environ[k]
            else:
                os.environ[k] = v


def walk_callable(node: ast.AST) -> Generator[ast.AST, Any, None]:
    """Similar to ``ast.walk``, but walks only function body and skips nested
    functions defined within the node.
    """
    todo: Deque[ast.AST] = deque([node])
    walked_func_def = False
    while todo:
        node = todo.popleft()
        if isinstance(node, ast.FunctionDef):
            if walked_func_def:
                continue
            walked_func_def = True
        todo.extend(ast.iter_child_nodes(node))
        yield node


_generator_callbacks_cache = LocalWeakReferencedCache(limit=128)


def is_generator_with_return_value(callable: Callable) -> bool:
    """
    Returns True if a callable is a generator function which includes a
    'return' statement with a value different than None, False otherwise
    """
    if callable in _generator_callbacks_cache:
        return bool(_generator_callbacks_cache[callable])

    def returns_none(return_node: ast.Return) -> bool:
        value = return_node.value
        return (
            value is None or isinstance(value, ast.NameConstant) and value.value is None
        )

    if inspect.isgeneratorfunction(callable):
        func = callable
        while isinstance(func, partial):
            func = func.func

        src = inspect.getsource(func)
        pattern = re.compile(r"(^[\t ]+)")
        code = pattern.sub("", src)

        match = pattern.match(src)  # finds indentation
        if match:
            code = re.sub(f"\n{match.group(0)}", "\n", code)  # remove indentation

        tree = ast.parse(code)
        for node in walk_callable(tree):
            if isinstance(node, ast.Return) and not returns_none(node):
                _generator_callbacks_cache[callable] = True
                return bool(_generator_callbacks_cache[callable])

    _generator_callbacks_cache[callable] = False
    return bool(_generator_callbacks_cache[callable])


def warn_on_generator_with_return_value(spider: "Spider", callable: Callable) -> None:
    """
    Logs a warning if a callable is a generator function and includes
    a 'return' statement with a value different than None
    """
    try:
        if is_generator_with_return_value(callable):
            warnings.warn(
                f'The "{spider.__class__.__name__}.{callable.__name__}" method is '
                'a generator and includes a "return" statement with a value '
                "different than None. This could lead to unexpected behaviour. Please see "
                "https://docs.python.org/3/reference/simple_stmts.html#the-return-statement "
                'for details about the semantics of the "return" statement within generators',
                stacklevel=2,
            )
    except IndentationError:
        callable_name = spider.__class__.__name__ + "." + callable.__name__
        warnings.warn(
            f'Unable to determine whether or not "{callable_name}" is a generator with a return value. '
            "This will not prevent your code from working, but it prevents Scrapy from detecting "
            f'potential issues in your implementation of "{callable_name}". Please, report this in the '
            "Scrapy issue tracker (https://github.com/scrapy/scrapy/issues), "
            f'including the code of "{callable_name}"',
            stacklevel=2,
        )