Python: get string representation of PyObject?

Question

I’ve got a C python extension, and I would like to print out some diagnostics.

I’m receiving a string as a PyObject*.

What’s the canonical way to obtain a string representation of this object, such that it usable as a const char *?

Asked By: Mark Harrison

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Source

Answer 1

Try PyObject_Repr (to mimic Python’s repr) or PyObject_Str (to mimic Python’s str).

Docs:

Compute a string representation of
object o. Returns the string
representation on success, NULL on
failure. This is the equivalent of the
Python expression repr(o). Called by
the repr() built-in function.

Answered By: Alexander Gessler

Answer 2

Use PyObject_Repr (to mimic Python’s repr function) or PyObject_Str (to mimic str), and then call PyString_AsString to get char * (you can, and usually should, use it as const char*, for example:

PyObject* objectsRepresentation = PyObject_Repr(yourObject);
const char* s = PyString_AsString(objectsRepresentation);

This method is OK for any PyObject. If you are absolutely sure yourObject is a Python string and not something else, like for instance a number, you can skip the first line and just do:

const char* s = PyString_AsString(yourObject);

Answered By: piokuc

Answer 3

Here is the correct answer if you are using Python 3:

static void reprint(PyObject *obj) {
    PyObject* repr = PyObject_Repr(obj);
    PyObject* str = PyUnicode_AsEncodedString(repr, "utf-8", "~E~");
    const char *bytes = PyBytes_AS_STRING(str);

    printf("REPR: %sn", bytes);

    Py_XDECREF(repr);
    Py_XDECREF(str);
}

Answered By: Romuald Brunet

Answer 4

If you need just print the object in Python 3 you can use one of these functions:

static void print_str(PyObject *o)
{
    PyObject_Print(o, stdout, Py_PRINT_RAW);
}

static void print_repr(PyObject *o)
{
    PyObject_Print(o, stdout, 0);
}

Answered By: Maxim Khrisanfov

Answer 5

PyObject *module_name;
PyUnicode_AsUTF8(module_name)

Answered By: Pulller

Answer 6

For python >=3.3:

char* str = PyUnicode_1BYTE_DATA(py_object);

Yes, this is a non-const pointer, you can potentially modify the (immutable) string via it.

Answered By: Mikhail

Answer 7

For an arbitrary PyObject*, first call
PyObject_Repr() or PyObject_Str() to get a PyUnicode* object.

In Python 3.3 and up, call PyUnicode_AsUTF8AndSize. In addition to the Python string you want a const char * for, this function takes an optional address to store the length in.

Python strings are objects with explicit length fields that may contain null bytes, while a const char* by itself is typically a pointer to a null-terminated C string. Converting a Python string to a C string is a potentially lossy operation. For that reason, all the other Python C-API functions that could return a const char* from a string are deprecated.

If you do not care about losing a bunch of the string if it happens to contain an embedded null byte, you can pass NULL for the size argument. For example,

PyObject* foo = PyUnicode_FromStringAndSize("foobar", 7);

printf("As const char*, ignoring length: %sn",
    PyUnicode_AsUTF8AndSize(foo, NULL));

prints

As const char*, ignoring length: foo

But you can also pass in the address of a size variable, to use with the const char*, to make sure that you’re getting the entire string.

PyObject* foo = PyUnicode_FromStringAndSize("foobar", 7);

printf("Including size: ");
size_t size;
const char* data = PyUnicode_AsUTF8AndSize(foo, &size);
fwrite(data, sizeof(data[0]), size, stdout);
putchar('n');

On my terminal, that outputs

$ ./main | cat -v
Including size: foo^@bar

Answered By: andrewdotn

Python: get string representation of PyObject?

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