The language reference carves out a special case for decimal zero literals: they may have leading “0” digits. Non-zero decimal literals may not. This is apparently deliberate:

    Note that leading zeros in a non-zero decimal number are not allowed. This is for disambiguation with C-style octal literals, which Python used before version 3.0.

    reference/lexical_analysis.html#integer-literals

But the expressed rationale (“for disambiguation with C-style octal literals”) does not explain making decimal zero special compared with non-zero.

Is there a good reason for this inconsistency::

    0000    # valid syntax for zero literal
    0003    # SyntaxError
    0123    # SyntaxError

To my reading, they should all cause SyntaxError. What is the rationale for the special case of the first one?

This was raised and rejected here:

http://bugs.python.org/issue24668

Short answer:  rationale unknown, but any number of zeroes is still unambiguously zero.

Closing as duplicate.

> rationale unknown

That's quite a pity. The inconsistency catches people out, and the docs do not make clear why an exception is made.

> but any number of zeroes is still unambiguously zero.

Of course. But the same is true for “0003 is unambiguously three”, yet that's invalid syntax.

I won't contest closing this report. But the above is not an explanation for the inconsistency.