0xC0000022L 0xC0000022L - 1 year ago 77
Python Question

Having trouble to understand the output in a Jupyter notebook ("Learning Cython")

In the O'Reilly video tutorial "Learning Cython" in chapter 5, there's a notebook called


First cell loads the Cython extension:

%load_ext cython

Followed by this Cython cell:

# cython: language_level=3

def f(char* text):

Then the following cell is used to demonstrate that a (Unicode) string cannot be used as

f('It is I, Arthur, son of Uther Pendragon')

The outcome here is a

All of the above is what I'd expect from the author's remarks in the voice-over. However, the outcome of the next cell:

f(b'It is I, Arthur, son of Uther Pendragon')

was this:

b'It is I, Arthur, son of Uther Pendragon'

and that stumped me.

Having used a plain
in the function
, why does the output appear as if it was run through
first, when inside the Cython code above it clearly was not run through

The author doesn't even mention this somewhat (at least to me) unexpected result in the voice-over.

What gives? Why does the output look like it was first passed through
? Are byte strings in Python 3 not "printable" (i.e. without
method) and therefore fall back to

PS: I have to admit I'm coming from Python 2.x and haven't had too much exposure to Python 3.x, so perhaps the difference is therein.

Jim Jim
Answer Source

Because it was. In Python 3, bytes_str uses bytes_repr internally:

static PyObject *
bytes_str(PyObject *op)
    if (Py_BytesWarningFlag) {
        if (PyErr_WarnEx(PyExc_BytesWarning,
                         "str() on a bytes instance", 1))
            return NULL;
    return bytes_repr(op);  // call repr on it

As such, print will, in essence, call repr(bytes_instance).

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