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Update mypy error messages in code examples, part 1 #7706

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6e3523b
revisited mypy error messages in additional_features.rst
hoefling Oct 13, 2019
5b23ebd
revisited mypy error messages in casts.rst
hoefling Oct 13, 2019
49f5059
revisited mypy error messages in class_basics.rst
hoefling Oct 13, 2019
bd24079
revisited mypy error messages in common_issues.rst
hoefling Oct 13, 2019
f542adb
changed code example where mypy reports incompatible types
hoefling Oct 13, 2019
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revisited mypy error messages in common_issues.rst 
Signed-off-by: Oleg Höfling <[email protected]>
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@hoefling
hoefling committed Oct 13, 2019
commit bd24079bc513f5f1a631e3b8169f8b4bb7a51ddc
14 changes: 7 additions & 7 deletions docs/source/common_issues.rst
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Expand Up @@ -47,8 +47,7 @@ flagged as an error.
.. code-block:: python

def foo(a: str) -> str:
return '(' + a.split() + ')'
# error: Unsupported operand types for + ("str" and List[str])
return '(' + a.split() + ')' # error: Unsupported operand types for + ("str" and "List[str]")

If you don't know what types to add, you can use ``Any``, but beware:

Expand Down Expand Up @@ -110,7 +109,7 @@ module:

.. code-block:: python

import frobnicate # Error: No module "frobnicate"
import frobnicate # error: Cannot find module named 'frobnicate'
frobnicate.start()

You can add a ``# type: ignore`` comment to tell mypy to ignore this
Expand Down Expand Up @@ -234,7 +233,7 @@ with the ``Any`` type.
def f() -> None:
n = 1
...
n = 'x' # Type error: n has type int
n = 'x' # error: Incompatible types in assignment (expression has type "str", variable has type "int")

.. note::

Expand Down Expand Up @@ -270,7 +269,8 @@ unexpected errors when combined with type inference. For example:

lst = [A(), A()] # Inferred type is List[A]
new_lst = [B(), B()] # inferred type is List[B]
lst = new_lst # mypy will complain about this, because List is invariant
# mypy will complain about this, because List is invariant
lst = new_lst # error: Incompatible types in assignment (expression has type "List[B]", variable has type "List[A]")
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@hoefling hoefling Oct 13, 2019

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Please advise whether keeping the explanation comments is useful, and whether the selected format is acceptable:
<code> # original explanation becomes

# original explanation
<code>  # mypy error

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@JukkaL JukkaL Oct 14, 2019

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This looks fine to me. Though here the explanation is pretty informal, and it may be better to be more explicit about what this refers to. Here is one idea:

    # Mypy rejects the following assignment, because List is invariant
    lst = new_lst  # error: ...


Possible strategies in such situations are:

Expand Down Expand Up @@ -611,7 +611,7 @@ Consider this example:
class C:
x = 42
c = C()
fun(c) # This is not safe
fun(c) # This is not safe: # error: Argument 1 to "fun" has incompatible type "C"; expected "P"
c.x << 5 # Since this will fail!

To work around this problem consider whether "mutating" is actually part
Expand Down Expand Up @@ -646,7 +646,7 @@ method signature. E.g.:
class Message:
def bytes(self):
...
def register(self, path: bytes): # error: Invalid type "mod.Message.bytes"
def register(self, path: bytes): # error: Function "__main__.Message.bytes" is not valid as a type
...

The third line elicits an error because mypy sees the argument type
Expand Down