如何符合mypy类型“SupportsWrite[str]”的要求？

Question

我有一个“智能”打开函数，可以打开各种文件并返回IO-ish对象类型：

def sopen(anything_at_all: str, mode: str) -> FileIO:
   ...

我在打印语句中使用它，如：

with sopen('footxt.gz', mode = 'w+') as fout:
    print("hello, world!", file=fout)

---

然后，当使用mypy0.812分析此代码时，我得到以下神秘错误：

Argument "fout" to "print" has incompatible type "FileIO"; expected "Optional[SupportsWrite[str]]"

好的，很好：SupportsWrite肯定比FileIO好，只有一个问题：当我调整代码以支持使用_typeshed.SupportsWrite写时，没有什么比这更好了

def sopen(anything_at_all: str, mode: str) \
  -> Union[SupportsWrite[str],SupportsRead[str]]:
  ...

mypy想要确切地说Optional[SupportsWrite]：

 Argument "fout" to "print" has incompatible type "Union[SupportsWrite[str], SupportsRead[str]]"; expected "Optional[SupportsWrite[str]]"

---

接下来，我尝试铸造，并创建某种类型的强制强制执行，但是在解释器中试用我的脚轮的过程中，当

>>> from _typeshed import SupportsRead, SupportsWrite
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named '_typeshed'

现在的根本问题是：在这种情况下，人们如何遵从mypy的意愿

Answer 1

TL；DR使用typing.IO代替FileIOtyping.IO支持内置open可能返回的所有返回类型

---

print本身将其file参数注释为Optional[SupportsWrite[str]]，因此mypy是正确的

要修复丢失的_typeshed模块错误（同样正确，它仅在类型检查时可用，而在解释器执行代码时不可用），可以使用if TYPE_CHECKING¹技巧，然后使用字符串注释

下面的几乎满足mypy：

from typing import Optional, TYPE_CHECKING

if TYPE_CHECKING:
    from _typeshed import SupportsWrite

def sopen(anything_at_all: str, mode: str) -> 'Optional[SupportsWrite[str]]':
   ...

with sopen('footxt.gz', mode = 'w+') as fout:
    print("hello, world!", file=fout)

用

test.py:9: error: Item "SupportsWrite[str]" of "Optional[SupportsWrite[str]]" has no attribute "__enter__"
test.py:9: error: Item "None" of "Optional[SupportsWrite[str]]" has no attribute "__enter__"
test.py:9: error: Item "SupportsWrite[str]" of "Optional[SupportsWrite[str]]" has no attribute "__exit__"
test.py:9: error: Item "None" of "Optional[SupportsWrite[str]]" has no attribute "__exit__"

输入typing.IO

您可以简单地使用typing.IO（这也恰好与open的返回类型匹配），而不是直接处理SupportsWrite。以下内容完全满足mypy：

from typing import IO

def sopen(anything_at_all: str, mode: str) -> IO:
   ...

with sopen('footxt.gz', mode = 'w+') as fout:
    print("hello, world!", file=fout)

---

¹TYPE_CHECKING是一个常量，默认情况下为False，并且仅由mypy和/或其他类型分析工具设置为True