Why hexa decode need two digit

October 8, 2024

The hex Python function produce 0x0 for the 0 number:

>>> hex(0)
'0x0'

But bytes.fromhexa only accept hexa with minimum of two digits:

>>> bytes.fromhex(hex(0)[2:])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: non-hexadecimal number found in fromhex() arg at position 1
>>> bytes.fromhex('00')
b'\x00'

Why bytes.fromhexa accept hexa from two digits and no with one digit ?

>Solution :

The fromhex() method is defined in the bytes class. You can find this in Objects/bytesobject.c. Here

PyObject *bytes_fromhex(PyObject *self, PyObject *arg) {
    // Convert argument to a string
    if (!PyArg_Parse(arg, "s", &hex_str)) {
        return NULL;
    }

    // Check if length of hex_str is even
    if (strlen(hex_str) % 2 != 0) {
        PyErr_SetString(PyExc_ValueError, "non-hexadecimal number found in fromhex() arg");
        return NULL;
    }

    // Conversion logic...
}

As you can see , the function first checks the length of the input string (hex_str). If the length of hex_str is not even (% 2 != 0), it raises a ValueError with a message indicating that there’s a non-hexadecimal number.

After the length check, the function proceeds to convert the hexadecimal string into bytes.

for (size_t i = 0; i < strlen(hex_str); i += 2) {
    // Process pairs of hex characters...
}

so from this logic

If you provided a single digit (like 0), it wouldn’t know how to interpret it. It could be thought of as incomplete since there isn’t a second digit to complete the byte representation.

if bytes.fromhex('0') were allowed, Python would have to guess how to pad it (as 00 or 0F), which introduces ambiguity.