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Syntax¶

Generated Thu 22 May 2025 13:57:03 UTC

Literals¶

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 Should not work	11 1

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
0
<string>:1: SyntaxWarning: invalid decimal literal
1
<string>:1: SyntaxWarning: invalid decimal literal
<string>:1: SyntaxWarning: invalid decimal literal
1
b'\x01'

Should have worked
Should have worked
Should have worked
Should have worked

Operators¶

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 SyntaxError: assignment expression cannot rebind comprehension iteration variable 'j'	[[0, 1], [0, 1]]

Unicode¶

Unicode name escapes are not implemented¶

Sample code:

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

CPy output:	uPy output:
a	NotImplementedError: unicode name escapes

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