docs/differences/syntax: Capture differences for ":=" operator.
Syntax
======
+Operators
+---------
+
+.. _cpydiff_syntax_assign_expr:
+
+MicroPython allows using := to assign to the variable of a comprehension, CPython raises a SyntaxError.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Cause:** MicroPython is optimised for code size and doesn't check this case.
+
+**Workaround:** Do not rely on this behaviour if writing CPython compatible code.
+
+Sample code::
+
+ print([i := -1 for i in range(4)])
+
++--------------------------------------------+----------------------+
+| CPy output: | uPy output: |
++--------------------------------------------+----------------------+
+| :: | :: |
+| | |
+| File "<stdin>", line 7 | [-1, -1, -1, -1] |
+| print([i := -1 for i in range(4)]) | |
+| ^ | |
+| SyntaxError: invalid syntax | |
++--------------------------------------------+----------------------+
+
Spaces
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