Print a C string in Python

Question

I want to print a C string using the Python print statement. The array vendorName contains the ASCIIZ C string A ANTHONY & SONS INC. My motivation is to convert the C string to a Python string such that I can use all of the Python string methods.

I have a struct:

class _vendrRecord(Structure):
    _pack_ = 1                                            # pack the struct
    _fields_ = [
        ("vendorName"            ,c_ubyte *(40 + 1)),
        ("ytdPayments"           ,c_ulong),
        ]

I want to print the string "vendorName" which is ASCIIZ.

I can print it using printf like this:

printf(b"%s\n", vendrRecord.vendorName)

I have tried this print(vendrRecord.vendorName) but it just prints the address. Based on information from Jamie Nicholl-Shelley, I tried print(cast(vendrRecord.vendorName,c_char_p).value) but that gives b'A ANTHONY & SONS INC'. I want simply A ANTHONY & SONS INC

Note that print(vendrRecord.ytdPayments) prints correctly.

Can't you access the string with vendrRecord.vendorName.content? I'm not so much into ctypes though. — Niklas R
– Niklas R, Commented Jan 9, 2016 at 21:38
vendrRecord.vendorName.content gives AttributeError: 'c_ubyte_Array_41' object has no attribute 'content' — Eddy
– Eddy, Commented Jan 10, 2016 at 13:08

DSM · Accepted Answer · 2016-01-10 14:16:44Z

4

It looks like you've figured out most of it and are just confused about string encoding. You've managed to get to a bytes object:

>>> v.vendorName
<__main__.c_ubyte_Array_41 object at 0xb0994a04>
>>> cast(v.vendorName, c_char_p)
c_char_p(176882328)
>>> cast(v.vendorName, c_char_p).value
b'A ANTHONY & SONS INC'

The bytes object is what it sounds like, just a sequence of bytes -- it's only as a convenience for the user that it's shown in a stringlike way. Unlike a string, each component is an integer:

>>> cast(v.vendorName, c_char_p).value[7]
78

But since we know that we're using ascii, we can decode this bytes object into a Python string, and then use all of the usual methods on it:

>>> s = cast(v.vendorName, c_char_p).value.decode("ascii")
>>> s
'A ANTHONY & SONS INC'
>>> type(s)
<class 'str'>
>>> s.lower()
'a anthony & sons inc'

answered Jan 10, 2016 at 14:16

DSM

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2 Comments

Eddy Over a year ago

Perfect! This does exactly what I want. It is interesting that your Python prints s.lower() with quotes but mine does not. I'm using the latest 32 bit version.

DSM Over a year ago

@Eddy: I didn't print anything. Since I was working at the console, it showed me the repr. If I'd done print(s.lower()), there would have been no quotes.

Jamie Nicholl-Shelley · Accepted Answer · 2016-01-10 05:40:57Z

1

You are printing the address as that is what you have asked the code to do. You have told it that it is a string type, but not declared it as a string in the function, instead use a pointer to the data at the address

Compared to print(), which figures this out for you as it a c++ based function

answered Jan 10, 2016 at 5:40

Jamie Nicholl-Shelley

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2 Comments

Eddy Over a year ago

That information got me closer but still not all the way. print(cast(vendrRecord.vendorName,c_char_p)) gives c_char_p(b'A ANTHONY & SONS INC'). How do I dereference it such that it is now a Python string? So I tried print(cast(vendrRecord.vendorName,c_char_p).value) but that gives b'A ANTHONY & SONS INC' (note the b).

Jamie Nicholl-Shelley Over a year ago

Sorry I don't do much Python now days. I imagine it's an output type identifier that's used to link with the python compiler.

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