Document the trouble with lambdas in the guides (#2105)

Author: wyattscarpenter

docs/guides/index.rst

@@ -12,6 +12,7 @@ Type System Guides
    :caption: Contents:
 
    libraries
+   lambda
    writing_stubs
    modernizing
    unreachable

docs/guides/lambda.rst

@@ -0,0 +1,88 @@
+*****************************************
+The Trouble (Or Lack Thereof) With Lambda
+*****************************************
+
+Lambda expressions are a common and useful part of the Python programming language.
+However, there is one problem with them: syntactically, they do not allow for type
+annotations. While it is perfectly simple to write ``lambda x: x``, you cannot directly
+indicate a type for x. (Type annotations are indicated by a colon, and so is the end
+of the lambda parameter list. Where would the type annotation go?)
+
+However, despite this infelicity, lambda expressions are not immune from static typing,
+and in fact follow the same static type rules as everything else. Type checkers try
+to deduce the type of the lambda arguments and return value, and if they can't they
+fall back to ``Any``. Due to the inability to directly indicate types for these,
+``Any`` tends to pop up quite often here. This means that many type errors may occur
+here unnoticed, which is bad. For instance, the following example is a runtime type
+error, but is uncaught in most (perhaps all) type checkers:
+
+.. code-block:: python
+
+   f1 = lambda a, b: a + b
+   f1(1, "a")
+
+(The alternative way of writing this, ``(lambda a, b: a + b)(1, "a")``, is typically
+caught by type checkers, because it is simple and immediate enough that they are able
+to deduce that a type error will occur.)
+
+..
+   (This is an RST comment.)
+   A slightly more realistic example of an uncaught lambda type error is
+
+   .. code-block :: python
+      def apply(f, *x):
+         f(*x)
+      apply((lambda a, b: a + b), 1, "a")
+
+   since it doesn't immediately defeat the purpose of a lambda by binding it.
+   It also fails to get caught by mypy and pyright in their default modes, as
+   required for the example. However, it's a little bit harder to understand,
+   so we went with the other one.
+
+There are some workarounds to this problem, which all involve assigning the lambda to
+something, in one way or another, and annotating that. This is a bit unfortunate,
+because the idiomatic use of a lambda involves not doing that. In fact, at that point
+you might as well just define a normal function. Let's call that our first workaround.
+
+``def f(x: object) -> object: return x``
+
+The second workaround is equivalent: assigning the lambda to a variable, and annotating
+the type of that variable with a Callable.
+
+``f: Callable[[object], object] = lambda x: x``
+
+..
+   (This is an RST comment. The following paragraph has been excised from the guide,
+   as most beginners will not know what a type comment is anyway — especially a function
+   type comment. However, the paragraph is left in this comment for greater context for
+   you, the future editor:)
+
+   Type comments on function definitions do not actually work on lambda, nor do
+   normal type comments help (although you can use a type comment on an assignment
+   to a variable with a lambda, of course; however this will have to be the Callable
+   syntax and not the function-arrow special one).
+
+Most type checkers include an option to emit a warning if they aren't able to deduce
+the type of an expression; this should be helpful if you want to avoid silent uncaught
+type errors resulting from lambda expressions being deduced as ``Any``. For instance,
+Mypy includes ``disallow_any_expr``/``--disallow-any-expr`` and Pyright includes
+``reportUnknownLambdaType``. Both of those options are set to true in the respective
+strict modes of those type checkers.
+
+In conclusion:
+
+1. There is no way to explicitly annotate lambda arguments or return values in the
+lambdas themselves.
+
+2. However, static typing rules still apply to lambdas, including type deduction.
+
+3. Many lambdas get deduced as ``Any``, which might suppress the reporting of other
+type errors.
+
+4. However, many lambdas get deduced fine, and for those it's not a problem.
+
+5. If you want to annotate the type of lambdas, you can bind them and annotate them
+there.
+
+6. Most type checkers have a setting that will warn you if anything gets deduced as
+``Any``, and you can use that to avoid false negatives relating to lambda.

docs/index.rst

@@ -23,6 +23,7 @@ Guides
    :maxdepth: 1
 
    guides/libraries
+   guides/lambda
    guides/writing_stubs
    guides/modernizing
    guides/unreachable