.. This document was generated by tools/gen-cpydiff.py

Syntax
======


Generated Thu 19 Jun 2025 07:21:24 UTC

Literals
--------

.. _cpydiff_syntax_literal_underscore:

MicroPython accepts underscores in numeric literals where CPython doesn't
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Cause:** Different parser implementation

MicroPython's tokenizer ignores underscores in numeric literals, while CPython
rejects multiple consecutive underscores and underscores after the last digit.

**Workaround:** Remove the underscores not accepted by CPython.

Sample code::

    
    try:
        print(eval("1__1"))
    except SyntaxError:
        print("Should not work")
    try:
        print(eval("1_"))
    except SyntaxError:
        print("Should not work")

+---------------------+-------------+
| CPy output:         | uPy output: |
+---------------------+-------------+
| ::                  | ::          |
|                     |             |
|     Should not work |     11      |
|     Should not work |     1       |
+---------------------+-------------+

.. _cpydiff_syntax_spaces:

MicroPython requires spaces between literal numbers and keywords or ".", CPython doesn't
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Cause:** Different parser implementation

MicroPython's tokenizer treats a sequence like ``1and`` as a single token, while CPython treats it as two tokens.

Since CPython 3.11, when the literal number is followed by a token, this syntax causes a ``SyntaxWarning`` for an "invalid literal". When a literal number is followed by a "." denoting attribute access, CPython does not warn.

**Workaround:** Add a space between the integer literal and the intended next token.

This also fixes the ``SyntaxWarning`` in CPython.

Sample code::

    
    try:
        print(eval("1and 0"))
    except SyntaxError:
        print("Should have worked")
    try:
        print(eval("1or 0"))
    except SyntaxError:
        print("Should have worked")
    try:
        print(eval("1if 1else 0"))
    except SyntaxError:
        print("Should have worked")
    try:
        print(eval("0x1.to_bytes(1)"))
    except SyntaxError:
        print("Should have worked")

+--------------------------------------------------------+------------------------+
| CPy output:                                            | uPy output:            |
+--------------------------------------------------------+------------------------+
| ::                                                     | ::                     |
|                                                        |                        |
|     <string>:1: SyntaxWarning: invalid decimal literal |     Should have worked |
|     0                                                  |     Should have worked |
|     <string>:1: SyntaxWarning: invalid decimal literal |     Should have worked |
|     1                                                  |     Should have worked |
|     <string>:1: SyntaxWarning: invalid decimal literal |                        |
|     <string>:1: SyntaxWarning: invalid decimal literal |                        |
|     1                                                  |                        |
|     b'\x01'                                            |                        |
+--------------------------------------------------------+------------------------+

Operators
---------

.. _cpydiff_syntax_assign_expr:

MicroPython allows := to assign to the iteration variable in nested comprehensions, CPython does not.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Cause:** MicroPython is optimised for code size. Although it is a syntax error to assign to the iteration variable in a standard comprehension (same as CPython), it doesn't check if an inner nested comprehension assigns to the iteration variable of the outer comprehension.

**Workaround:** Do not use := to assign to the iteration variable of a comprehension.

Sample code::

    
    print([[(j := i) for i in range(2)] for j in range(2)])

+-------------------------------------------------------------------------------------------+----------------------+
| CPy output:                                                                               | uPy output:          |
+-------------------------------------------------------------------------------------------+----------------------+
| ::                                                                                        | ::                   |
|                                                                                           |                      |
|       File "<stdin>", line 8                                                              |     [[0, 1], [0, 1]] |
|     SyntaxError: assignment expression cannot rebind comprehension iteration variable 'j' |                      |
+-------------------------------------------------------------------------------------------+----------------------+

Unicode
-------

.. _cpydiff_syntax_unicode_nameesc:

Unicode name escapes are not implemented
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sample code::

    
    print("\N{LATIN SMALL LETTER A}")

+-------------+-----------------------------------------------+
| CPy output: | uPy output:                                   |
+-------------+-----------------------------------------------+
| ::          | ::                                            |
|             |                                               |
|     a       |     NotImplementedError: unicode name escapes |
+-------------+-----------------------------------------------+

Unpacking
---------

.. _cpydiff_syntax_arg_unpacking:

Argument unpacking does not work if the argument being unpacked is the nth or greater argument where n is the number of bits in an MP_SMALL_INT.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Cause:** The implementation uses an MP_SMALL_INT to flag args that need to be unpacked.

**Workaround:** Use fewer arguments.

Sample code::

    
    
    def example(*args):
        print(len(args))
    
    
    MORE = ["a", "b", "c"]
    
    example(
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
        16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
        32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
        48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
        *MORE,
    )
    

+-------------+--------------------------------------------+
| CPy output: | uPy output:                                |
+-------------+--------------------------------------------+
| ::          | ::                                         |
|             |                                            |
|     67      |     Traceback (most recent call last):     |
|             |       File "<stdin>", line 21, in <module> |
|             |     SyntaxError: too many args             |
+-------------+--------------------------------------------+