# Result
[](https://github.com/rustedpy/result/actions/workflows/ci.yml?query=branch%3Amaster)
[](https://codecov.io/gh/rustedpy/result)
A simple Result type for Python 3 [inspired by
Rust](https://doc.rust-lang.org/std/result/), fully type annotated.
## Installation
Latest release:
``` sh
$ pip install result
```
Latest GitHub `master` branch version:
``` sh
$ pip install git+https://github.com/rustedpy/result
```
## Summary
The idea is that a result value can be either `Ok(value)` or
`Err(error)`, with a way to differentiate between the two. `Ok` and
`Err` are both classes encapsulating an arbitrary value. `Result[T, E]`
is a generic type alias for `typing.Union[Ok[T], Err[E]]`. It will
change code like this:
``` python
def get_user_by_email(email: str) -> Tuple[Optional[User], Optional[str]]:
"""
Return the user instance or an error message.
"""
if not user_exists(email):
return None, 'User does not exist'
if not user_active(email):
return None, 'User is inactive'
user = get_user(email)
return user, None
user, reason = get_user_by_email('ueli@example.com')
if user is None:
raise RuntimeError('Could not fetch user: %s' % reason)
else:
do_something(user)
```
To something like this:
``` python
from result import Ok, Err, Result, is_ok, is_err
def get_user_by_email(email: str) -> Result[User, str]:
"""
Return the user instance or an error message.
"""
if not user_exists(email):
return Err('User does not exist')
if not user_active(email):
return Err('User is inactive')
user = get_user(email)
return Ok(user)
user_result = get_user_by_email(email)
if isinstance(user_result, Ok): # or `is_ok(user_result)`
# type(user_result.ok_value) == User
do_something(user_result.ok_value)
else: # or `elif is_err(user_result)`
# type(user_result.err_value) == str
raise RuntimeError('Could not fetch user: %s' % user_result.err_value)
```
Note that `.ok_value` exists only on an instance of `Ok` and
`.err_value` exists only on an instance of `Err`.
And if you're using python version `3.10` or later, you can use the
elegant `match` statement as well:
``` python
from result import Result, Ok, Err
def divide(a: int, b: int) -> Result[int, str]:
if b == 0:
return Err("Cannot divide by zero")
return Ok(a // b)
values = [(10, 0), (10, 5)]
for a, b in values:
match divide(a, b):
case Ok(value):
print(f"{a} // {b} == {value}")
case Err(e):
print(e)
```
Not all methods
(<https://doc.rust-lang.org/std/result/enum.Result.html>) have been
implemented, only the ones that make sense in the Python context. By
using `isinstance` to check for `Ok` or `Err` you get type safe access
to the contained value when using [MyPy](https://mypy.readthedocs.io/)
to typecheck your code. All of this in a package allowing easier
handling of values that can be OK or not, without resorting to custom
exceptions.
## API
Auto generated API docs are also available at
[./docs/README.md](./docs/README.md).
Creating an instance:
``` python
>>> from result import Ok, Err
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
```
Checking whether a result is `Ok` or `Err`. You can either use `is_ok`
and `is_err` type guard **functions** or `isinstance`. This way you get
type safe access that can be checked with MyPy. The `is_ok()` or
`is_err()` **methods** can be used if you don't need the type safety
with MyPy:
``` python
>>> res = Ok('yay')
>>> isinstance(res, Ok)
True
>>> is_ok(res)
True
>>> isinstance(res, Err)
False
>>> is_err(res)
False
>>> res.is_ok()
True
>>> res.is_err()
False
```
You can also check if an object is `Ok` or `Err` by using the `OkErr`
type. Please note that this type is designed purely for convenience, and
should not be used for anything else. Using `(Ok, Err)` also works fine:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> isinstance(res1, OkErr)
True
>>> isinstance(res2, OkErr)
True
>>> isinstance(1, OkErr)
False
>>> isinstance(res1, (Ok, Err))
True
```
Convert a `Result` to the value or `None`:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.ok()
'yay'
>>> res2.ok()
None
```
Convert a `Result` to the error or `None`:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.err()
None
>>> res2.err()
'nay'
```
Access the value directly, without any other checks:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.ok_value
'yay'
>>> res2.err_value
'nay'
```
Note that this is a property, you cannot assign to it. Results are
immutable.
When the value inside is irrelevant, we suggest using `None` or a
`bool`, but you're free to use any value you think works best. An
instance of a `Result` (`Ok` or `Err`) must always contain something. If
you're looking for a type that might contain a value you may be
interested in a [maybe](https://github.com/rustedpy/maybe).
The `unwrap` method returns the value if `Ok` and `unwrap_err` method
returns the error value if `Err`, otherwise it raises an `UnwrapError`:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap()
'yay'
>>> res2.unwrap()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\project\result\result.py", line 107, in unwrap
return self.expect("Called `Result.unwrap()` on an `Err` value")
File "C:\project\result\result.py", line 101, in expect
raise UnwrapError(message)
result.result.UnwrapError: Called `Result.unwrap()` on an `Err` value
>>> res1.unwrap_err()
Traceback (most recent call last):
...
>>>res2.unwrap_err()
'nay'
```
A custom error message can be displayed instead by using `expect` and
`expect_err`:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.expect('not ok')
'yay'
>>> res2.expect('not ok')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\project\result\result.py", line 101, in expect
raise UnwrapError(message)
result.result.UnwrapError: not ok
>>> res1.expect_err('not err')
Traceback (most recent call last):
...
>>> res2.expect_err('not err')
'nay'
```
A default value can be returned instead by using `unwrap_or` or
`unwrap_or_else`:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap_or('default')
'yay'
>>> res2.unwrap_or('default')
'default'
>>> res1.unwrap_or_else(str.upper)
'yay'
>>> res2.unwrap_or_else(str.upper)
'NAY'
```
The `unwrap` method will raised an `UnwrapError`. A custom exception can
be raised by using the `unwrap_or_raise` method instead:
``` python
>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap_or_raise(ValueError)
'yay'
>>> res2.unwrap_or_raise(ValueError)
ValueError: nay
```
Values and errors can be mapped using `map`, `map_or`, `map_or_else` and
`map_err`:
``` python
>>> Ok(1).map(lambda x: x + 1)
Ok(2)
>>> Err('nay').map(lambda x: x + 1)
Err('nay')
>>> Ok(1).map_or(-1, lambda x: x + 1)
2
>>> Err(1).map_or(-1, lambda x: x + 1)
-1
>>> Ok(1).map_or_else(lambda: 3, lambda x: x + 1)
2
>>> Err('nay').map_or_else(lambda: 3, lambda x: x + 1)
3
>>> Ok(1).map_err(lambda x: x + 1)
Ok(1)
>>> Err(1).map_err(lambda x: x + 1)
Err(2)
```
To save memory, both the `Ok` and `Err` classes are ‘slotted’, i.e. they
define `__slots__`. This means assigning arbitrary attributes to
instances will raise `AttributeError`.
### `as_result` Decorator
The `as_result()` decorator can be used to quickly turn ‘normal’
functions into `Result` returning ones by specifying one or more
exception types:
``` python
@as_result(ValueError, IndexError)
def f(value: int) -> int:
if value == 0:
raise ValueError # becomes Err
elif value == 1:
raise IndexError # becomes Err
elif value == 2:
raise KeyError # raises Exception
else:
return value # becomes Ok
res = f(0) # Err[ValueError()]
res = f(1) # Err[IndexError()]
res = f(2) # raises KeyError
res = f(3) # Ok[3]
```
`Exception` (or even `BaseException`) can be specified to create a
‘catch all’ `Result` return type. This is effectively the same as `try`
followed by `except Exception`, which is not considered good practice in
most scenarios, and hence this requires explicit opt-in.
Since `as_result` is a regular decorator, it can be used to wrap
existing functions (also from other libraries), albeit with a slightly
unconventional syntax (without the usual `@`):
``` python
import third_party
x = third_party.do_something(...) # could raise; who knows?
safe_do_something = as_result(Exception)(third_party.do_something)
res = safe_do_something(...) # Ok(...) or Err(...)
if isinstance(res, Ok):
print(res.ok_value)
```
### Do notation
Do notation is syntactic sugar for a sequence of `and_then()` calls.
Much like the equivalent in Rust or Haskell, but with different syntax.
Instead of `x <- Ok(1)` we write `for x in Ok(1)`. Since the syntax is
generator-based, the final result must be the first line, not the last.
``` python
final_result: Result[int, str] = do(
Ok(x + y)
for x in Ok(1)
for y in Ok(2)
)
```
Note that if you exclude the type annotation,
`final_result: Result[float, int] = ...`, your type checker may be
unable to infer the return type. To avoid an errors or warnings from
your type checker, you should add a type hint when using the `do`
function.
This is similar to Rust's [m!
macro](https://docs.rs/do-notation/latest/do_notation/):
``` rust
use do_notation::m;
let r = m! {
x <- Some(1);
y <- Some(2);
Some(x + y)
};
```
Note that if your do statement has multiple <span
class="title-ref">for\`s, you can access an identifier bound in a
previous \`for</span>. Example:
``` python
my_result: Result[int, str] = do(
f(x, y, z)
for x in get_x()
for y in calculate_y_from_x(x)
for z in calculate_z_from_x_y(x, y)
)
```
You can use `do()` with awaited values as follows:
``` python
async def process_data(data) -> Result[int, str]:
res1 = await get_result_1(data)
res2 = await get_result_2(data)
return do(
Ok(x + y)
for x in res1
for y in res2
)
```
However, if you want to await something inside the expression, use
`do_async()`:
``` python
async def process_data(data) -> Result[int, str]:
return do_async(
Ok(x + y)
for x in await get_result_1(data)
for y in await get_result_2(data)
)
```
Troubleshooting `do()` calls:
``` python
TypeError("Got async_generator but expected generator")
```
Sometimes regular `do()` can handle async values, but this error means
you have hit a case where it does not. You should use `do_async()` here
instead.
## Contributing
These steps should work on any Unix-based system (Linux, macOS, etc) with Python
and `make` installed. On Windows, you will need to refer to the Python
documentation (linked below) and reference the `Makefile` for commands to run
from the non-unix shell you're using on Windows.
1. Setup and activate a virtual environment. See [Python docs][pydocs-venv] for more
information about virtual environments and setup.
2. Run `make install` to install dependencies
3. Switch to a new git branch and make your changes
4. Test your changes:
- `make test`
- `make lint`
- You can also start a Python REPL and import `result`
5. Update documentation
- Edit any relevant docstrings, markdown files
- Run `make docs`
6. Add an entry to the [changelog](./CHANGELOG.md)
5. Git commit all your changes and create a new PR.
[pydocs-venv]: https://docs.python.org/3/library/venv.html
## FAQ
- **Why do I get the "Cannot infer type argument" error with MyPy?**
There is [a bug in MyPy](https://github.com/python/mypy/issues/230)
which can be triggered in some scenarios. Using `if isinstance(res, Ok)`
instead of `if res.is_ok()` will help in some cases. Otherwise using
[one of these
workarounds](https://github.com/python/mypy/issues/3889#issuecomment-325997911)
can help.
## License
MIT License
Raw data
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"home_page": "https://github.com/rustedpy/result",
"name": "result-test-publish-version",
"maintainer": "rustedpy github org members (https://github.com/rustedpy)",
"docs_url": null,
"requires_python": ">=3.8",
"maintainer_email": "",
"keywords": "rust,result,enum",
"author": "Danilo Bargen",
"author_email": "mail@dbrgn.ch",
"download_url": "https://files.pythonhosted.org/packages/01/d9/7c83a9922717263dfc11af60d6a4cc00a196ffc7475c80ab56a921ecd452/result-test-publish-version-0.17.0.dev1.tar.gz",
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"description": "# Result\n\n[](https://github.com/rustedpy/result/actions/workflows/ci.yml?query=branch%3Amaster)\n[](https://codecov.io/gh/rustedpy/result)\n\nA simple Result type for Python 3 [inspired by\nRust](https://doc.rust-lang.org/std/result/), fully type annotated.\n\n## Installation\n\nLatest release:\n\n``` sh\n$ pip install result\n```\n\nLatest GitHub `master` branch version:\n\n``` sh\n$ pip install git+https://github.com/rustedpy/result\n```\n\n## Summary\n\nThe idea is that a result value can be either `Ok(value)` or\n`Err(error)`, with a way to differentiate between the two. `Ok` and\n`Err` are both classes encapsulating an arbitrary value. `Result[T, E]`\nis a generic type alias for `typing.Union[Ok[T], Err[E]]`. It will\nchange code like this:\n\n``` python\ndef get_user_by_email(email: str) -> Tuple[Optional[User], Optional[str]]:\n \"\"\"\n Return the user instance or an error message.\n \"\"\"\n if not user_exists(email):\n return None, 'User does not exist'\n if not user_active(email):\n return None, 'User is inactive'\n user = get_user(email)\n return user, None\n\nuser, reason = get_user_by_email('ueli@example.com')\nif user is None:\n raise RuntimeError('Could not fetch user: %s' % reason)\nelse:\n do_something(user)\n```\n\nTo something like this:\n\n``` python\nfrom result import Ok, Err, Result, is_ok, is_err\n\ndef get_user_by_email(email: str) -> Result[User, str]:\n \"\"\"\n Return the user instance or an error message.\n \"\"\"\n if not user_exists(email):\n return Err('User does not exist')\n if not user_active(email):\n return Err('User is inactive')\n user = get_user(email)\n return Ok(user)\n\nuser_result = get_user_by_email(email)\nif isinstance(user_result, Ok): # or `is_ok(user_result)`\n # type(user_result.ok_value) == User\n do_something(user_result.ok_value)\nelse: # or `elif is_err(user_result)`\n # type(user_result.err_value) == str\n raise RuntimeError('Could not fetch user: %s' % user_result.err_value)\n```\n\nNote that `.ok_value` exists only on an instance of `Ok` and\n`.err_value` exists only on an instance of `Err`.\n\nAnd if you're using python version `3.10` or later, you can use the\nelegant `match` statement as well:\n\n``` python\nfrom result import Result, Ok, Err\n\ndef divide(a: int, b: int) -> Result[int, str]:\n if b == 0:\n return Err(\"Cannot divide by zero\")\n return Ok(a // b)\n\nvalues = [(10, 0), (10, 5)]\nfor a, b in values:\n match divide(a, b):\n case Ok(value):\n print(f\"{a} // {b} == {value}\")\n case Err(e):\n print(e)\n```\n\nNot all methods\n(<https://doc.rust-lang.org/std/result/enum.Result.html>) have been\nimplemented, only the ones that make sense in the Python context. By\nusing `isinstance` to check for `Ok` or `Err` you get type safe access\nto the contained value when using [MyPy](https://mypy.readthedocs.io/)\nto typecheck your code. All of this in a package allowing easier\nhandling of values that can be OK or not, without resorting to custom\nexceptions.\n\n## API\n\nAuto generated API docs are also available at\n[./docs/README.md](./docs/README.md).\n\nCreating an instance:\n\n``` python\n>>> from result import Ok, Err\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n```\n\nChecking whether a result is `Ok` or `Err`. You can either use `is_ok`\nand `is_err` type guard **functions** or `isinstance`. This way you get\ntype safe access that can be checked with MyPy. The `is_ok()` or\n`is_err()` **methods** can be used if you don't need the type safety\nwith MyPy:\n\n``` python\n>>> res = Ok('yay')\n>>> isinstance(res, Ok)\nTrue\n>>> is_ok(res)\nTrue\n>>> isinstance(res, Err)\nFalse\n>>> is_err(res)\nFalse\n>>> res.is_ok()\nTrue\n>>> res.is_err()\nFalse\n```\n\nYou can also check if an object is `Ok` or `Err` by using the `OkErr`\ntype. Please note that this type is designed purely for convenience, and\nshould not be used for anything else. Using `(Ok, Err)` also works fine:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> isinstance(res1, OkErr)\nTrue\n>>> isinstance(res2, OkErr)\nTrue\n>>> isinstance(1, OkErr)\nFalse\n>>> isinstance(res1, (Ok, Err))\nTrue\n```\n\nConvert a `Result` to the value or `None`:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.ok()\n'yay'\n>>> res2.ok()\nNone\n```\n\nConvert a `Result` to the error or `None`:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.err()\nNone\n>>> res2.err()\n'nay'\n```\n\nAccess the value directly, without any other checks:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.ok_value\n'yay'\n>>> res2.err_value\n'nay'\n```\n\nNote that this is a property, you cannot assign to it. Results are\nimmutable.\n\nWhen the value inside is irrelevant, we suggest using `None` or a\n`bool`, but you're free to use any value you think works best. An\ninstance of a `Result` (`Ok` or `Err`) must always contain something. If\nyou're looking for a type that might contain a value you may be\ninterested in a [maybe](https://github.com/rustedpy/maybe).\n\nThe `unwrap` method returns the value if `Ok` and `unwrap_err` method\nreturns the error value if `Err`, otherwise it raises an `UnwrapError`:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.unwrap()\n'yay'\n>>> res2.unwrap()\nTraceback (most recent call last):\nFile \"<stdin>\", line 1, in <module>\nFile \"C:\\project\\result\\result.py\", line 107, in unwrap\n return self.expect(\"Called `Result.unwrap()` on an `Err` value\")\nFile \"C:\\project\\result\\result.py\", line 101, in expect\n raise UnwrapError(message)\nresult.result.UnwrapError: Called `Result.unwrap()` on an `Err` value\n>>> res1.unwrap_err()\nTraceback (most recent call last):\n...\n>>>res2.unwrap_err()\n'nay'\n```\n\nA custom error message can be displayed instead by using `expect` and\n`expect_err`:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.expect('not ok')\n'yay'\n>>> res2.expect('not ok')\nTraceback (most recent call last):\nFile \"<stdin>\", line 1, in <module>\nFile \"C:\\project\\result\\result.py\", line 101, in expect\n raise UnwrapError(message)\nresult.result.UnwrapError: not ok\n>>> res1.expect_err('not err')\nTraceback (most recent call last):\n...\n>>> res2.expect_err('not err')\n'nay'\n```\n\nA default value can be returned instead by using `unwrap_or` or\n`unwrap_or_else`:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.unwrap_or('default')\n'yay'\n>>> res2.unwrap_or('default')\n'default'\n>>> res1.unwrap_or_else(str.upper)\n'yay'\n>>> res2.unwrap_or_else(str.upper)\n'NAY'\n```\n\nThe `unwrap` method will raised an `UnwrapError`. A custom exception can\nbe raised by using the `unwrap_or_raise` method instead:\n\n``` python\n>>> res1 = Ok('yay')\n>>> res2 = Err('nay')\n>>> res1.unwrap_or_raise(ValueError)\n'yay'\n>>> res2.unwrap_or_raise(ValueError)\nValueError: nay\n```\n\nValues and errors can be mapped using `map`, `map_or`, `map_or_else` and\n`map_err`:\n\n``` python\n>>> Ok(1).map(lambda x: x + 1)\nOk(2)\n>>> Err('nay').map(lambda x: x + 1)\nErr('nay')\n>>> Ok(1).map_or(-1, lambda x: x + 1)\n2\n>>> Err(1).map_or(-1, lambda x: x + 1)\n-1\n>>> Ok(1).map_or_else(lambda: 3, lambda x: x + 1)\n2\n>>> Err('nay').map_or_else(lambda: 3, lambda x: x + 1)\n3\n>>> Ok(1).map_err(lambda x: x + 1)\nOk(1)\n>>> Err(1).map_err(lambda x: x + 1)\nErr(2)\n```\n\nTo save memory, both the `Ok` and `Err` classes are \u2018slotted\u2019, i.e. they\ndefine `__slots__`. This means assigning arbitrary attributes to\ninstances will raise `AttributeError`.\n\n### `as_result` Decorator\n\nThe `as_result()` decorator can be used to quickly turn \u2018normal\u2019\nfunctions into `Result` returning ones by specifying one or more\nexception types:\n\n``` python\n@as_result(ValueError, IndexError)\ndef f(value: int) -> int:\n if value == 0:\n raise ValueError # becomes Err\n elif value == 1:\n raise IndexError # becomes Err\n elif value == 2:\n raise KeyError # raises Exception\n else:\n return value # becomes Ok\n\nres = f(0) # Err[ValueError()]\nres = f(1) # Err[IndexError()]\nres = f(2) # raises KeyError\nres = f(3) # Ok[3]\n```\n\n`Exception` (or even `BaseException`) can be specified to create a\n\u2018catch all\u2019 `Result` return type. This is effectively the same as `try`\nfollowed by `except Exception`, which is not considered good practice in\nmost scenarios, and hence this requires explicit opt-in.\n\nSince `as_result` is a regular decorator, it can be used to wrap\nexisting functions (also from other libraries), albeit with a slightly\nunconventional syntax (without the usual `@`):\n\n``` python\nimport third_party\n\nx = third_party.do_something(...) # could raise; who knows?\n\nsafe_do_something = as_result(Exception)(third_party.do_something)\n\nres = safe_do_something(...) # Ok(...) or Err(...)\nif isinstance(res, Ok):\n print(res.ok_value)\n```\n\n### Do notation\n\nDo notation is syntactic sugar for a sequence of `and_then()` calls.\nMuch like the equivalent in Rust or Haskell, but with different syntax.\nInstead of `x <- Ok(1)` we write `for x in Ok(1)`. Since the syntax is\ngenerator-based, the final result must be the first line, not the last.\n\n``` python\nfinal_result: Result[int, str] = do(\n Ok(x + y)\n for x in Ok(1)\n for y in Ok(2)\n)\n```\n\nNote that if you exclude the type annotation,\n`final_result: Result[float, int] = ...`, your type checker may be\nunable to infer the return type. To avoid an errors or warnings from\nyour type checker, you should add a type hint when using the `do`\nfunction.\n\nThis is similar to Rust's [m!\nmacro](https://docs.rs/do-notation/latest/do_notation/):\n\n``` rust\nuse do_notation::m;\nlet r = m! {\n x <- Some(1);\n y <- Some(2);\n Some(x + y)\n};\n```\n\nNote that if your do statement has multiple <span\nclass=\"title-ref\">for\\`s, you can access an identifier bound in a\nprevious \\`for</span>. Example:\n\n``` python\nmy_result: Result[int, str] = do(\n f(x, y, z)\n for x in get_x()\n for y in calculate_y_from_x(x)\n for z in calculate_z_from_x_y(x, y)\n)\n```\n\nYou can use `do()` with awaited values as follows:\n\n``` python\nasync def process_data(data) -> Result[int, str]:\n res1 = await get_result_1(data)\n res2 = await get_result_2(data)\n return do(\n Ok(x + y)\n for x in res1\n for y in res2\n )\n```\n\nHowever, if you want to await something inside the expression, use\n`do_async()`:\n\n``` python\nasync def process_data(data) -> Result[int, str]:\n return do_async(\n Ok(x + y)\n for x in await get_result_1(data)\n for y in await get_result_2(data)\n )\n```\n\nTroubleshooting `do()` calls:\n\n``` python\nTypeError(\"Got async_generator but expected generator\")\n```\n\nSometimes regular `do()` can handle async values, but this error means\nyou have hit a case where it does not. You should use `do_async()` here\ninstead.\n\n## Contributing\n\nThese steps should work on any Unix-based system (Linux, macOS, etc) with Python\nand `make` installed. On Windows, you will need to refer to the Python\ndocumentation (linked below) and reference the `Makefile` for commands to run\nfrom the non-unix shell you're using on Windows.\n\n1. Setup and activate a virtual environment. See [Python docs][pydocs-venv] for more\n information about virtual environments and setup.\n2. Run `make install` to install dependencies\n3. Switch to a new git branch and make your changes\n4. Test your changes:\n - `make test`\n - `make lint`\n - You can also start a Python REPL and import `result`\n5. Update documentation\n - Edit any relevant docstrings, markdown files\n - Run `make docs`\n6. Add an entry to the [changelog](./CHANGELOG.md)\n5. Git commit all your changes and create a new PR.\n\n[pydocs-venv]: https://docs.python.org/3/library/venv.html\n\n## FAQ\n\n- **Why do I get the \"Cannot infer type argument\" error with MyPy?**\n\nThere is [a bug in MyPy](https://github.com/python/mypy/issues/230)\nwhich can be triggered in some scenarios. Using `if isinstance(res, Ok)`\ninstead of `if res.is_ok()` will help in some cases. Otherwise using\n[one of these\nworkarounds](https://github.com/python/mypy/issues/3889#issuecomment-325997911)\ncan help.\n\n## License\n\nMIT License\n",
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