Why does the size of this Python String change on a failed int conversion

From the tweet here:

import sys
x = 'ñ'
print(sys.getsizeof(x))
int(x) #throws an error
print(sys.getsizeof(x))

We get 74, then 77 bytes for the two getsizeof calls.

It looks like we are adding 3 bytes to the object, from the failed int call.

Some more examples from twitter (you may need to restart python to reset the size back to 74):

x = 'ñ'
y = 'ñ'
int(x)
print(sys.getsizeof(y))

77!

print(sys.getsizeof('ñ'))
int('ñ')
print(sys.getsizeof('ñ'))

74, then 77.

The code that converts strings to ints in CPython 3.6 requests a UTF-8 form of the string to work with:

buffer = PyUnicode_AsUTF8AndSize(asciidig, &buflen);

and the string creates the UTF-8 representation the first time it's requested and caches it on the string object:

if (PyUnicode_UTF8(unicode) == NULL) {
    assert(!PyUnicode_IS_COMPACT_ASCII(unicode));
    bytes = _PyUnicode_AsUTF8String(unicode, NULL);
    if (bytes == NULL)
        return NULL;
    _PyUnicode_UTF8(unicode) = PyObject_MALLOC(PyBytes_GET_SIZE(bytes) + 1);
    if (_PyUnicode_UTF8(unicode) == NULL) {
        PyErr_NoMemory();
        Py_DECREF(bytes);
        return NULL;
    }
    _PyUnicode_UTF8_LENGTH(unicode) = PyBytes_GET_SIZE(bytes);
    memcpy(_PyUnicode_UTF8(unicode),
              PyBytes_AS_STRING(bytes),
              _PyUnicode_UTF8_LENGTH(unicode) + 1);
    Py_DECREF(bytes);
}

The extra 3 bytes are for the UTF-8 representation.

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You might be wondering why the size doesn't change when the string is something like '40' or 'plain ascii text'. That's because if the string is in "compact ascii" representation, Python doesn't create a separate UTF-8 representation. It returns the ASCII representation directly, which is already valid UTF-8:

#define PyUnicode_UTF8(op)                              \
    (assert(_PyUnicode_CHECK(op)),                      \
     assert(PyUnicode_IS_READY(op)),                    \
     PyUnicode_IS_COMPACT_ASCII(op) ?                   \
         ((char*)((PyASCIIObject*)(op) + 1)) :          \
         _PyUnicode_UTF8(op))

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You also might wonder why the size doesn't change for something like '１'. That's U+FF11 FULLWIDTH DIGIT ONE, which int treats as equivalent to '1'. That's because one of the earlier steps in the string-to-int process is

asciidig = _PyUnicode_TransformDecimalAndSpaceToASCII(u);

which converts all whitespace characters to ' ' and converts all Unicode decimal digits to the corresponding ASCII digits. This conversion returns the original string if it doesn't end up changing anything, but when it does make changes, it creates a new string, and the new string is the one that gets a UTF-8 representation created.

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As for the cases where calling int on one string looks like it affects another, those are actually the same string object. There are many conditions under which Python will reuse strings, all just as firmly in Weird Implementation Detail Land as everything we've discussed so far. For 'ñ', the reuse happens because this is a single-character string in the Latin-1 range ('\x00'-'\xff'), and the implementation stores and reuses those.