I need to use in Python C-coded translation of string to reverse polish notation and calculation of it.
For this I do next:
import ctypes
input_string = bytes(memoryview("2 + 3.2 + 4".encode()))
result = str(self.model.functions['equal'](input_string))
where equal function is in:
def init_functions(self):
equal = self.c_lib.equal
equal.restype = ctypes.c_double
equal.argtypes = [ctypes.c_char_p]
self.functions.update({'equal': equal})
Where function in C is:
double equal(char *input) {
stack S = parseInput(input);
stack polish = toReversePolishNotation(&S);
return calculate(&polish);
}
It does calculation perfectly fine in C (checked with tests and memory leaks), always delivering the right answer.
But when I use this function in Python, it occasionally delivers some wrong output.
Sometimes it is 0, sometimes it is some strange number, sometimes it is endless cycle.
For example:
C result
Some random Python result
Strictly the next after random result in Python
More of it, my beloved example is "39.9 - 52.0". In C it is always as stated. But Python passes this string to C as "39.9 - 52.9".
I tried to set input_string = '' in Python, tried to memset char* input by '\0', tried to use different implementations of input_string:
input_string = ctypes.c_char_p("2 + 3.2 + 4".encode('utf-8'))
input_string = bytes("2 + 3.2 + 4", encoding='utf-8')
Python never delivers right answer consequently. It always fails. I don't know, if I need to free input_string somehow or something else is wrong.
The C code expects a pointer to mutable memory (probably for some tokenization). For that you need to use create_string_buffer.
input_string = ctypes.create_string_buffer(b"2 + 3.2 + 4")
See the documentation and tutorial
I use asterisk notation in regular python as:
>>> x=(10,11)
>>> y=(12,13)
>>> z=99
>>> print(*x)
10 11
>>> print(*x, *y, z)
10 11 12 13 99
But when I try to do similar in python mode of Processing, it gives me essentially a syntax error: processing.app.SketchException: Maybe there's an unclosed paren or quote mark somewhere before this line?
p1= (20,20)
p2=(40,40)
c1 = (15,15)
c2 = (50,50)
print(p1)
print(*p1)
# bezier(**p1, **c1, **c2, **p2)
Is this not supported in Processing.PY?
It works fine, I use it all the time.
p1 = (20, 20)
p2 = (40, 40)
translate(*p1) # this works
# you can't use "star" unpacking twice like this.
# line(*p1, *p2) # this won't work!
Beware that in Python 2 print is not a function, so I have used from __future__ import print_function to make your example work on the image bellow. The print function can handle an arbitrary number of positional arguments, but again, not * "star" unpacking twice.
I have noticed in your question example ** which can be used for
keyword arguments & dictionary unpacking. It also works fine overall, but it would not work with the bezier() comment you provided. The error message might have misled you.
I want to "unpack" OR de-serialize the formatted data that is outputed from python's struct.pack() function. The data is sent over the network to another platform that uses Java only.
The Python function that sends data over the network, uses this formater:
def formatOutputMsg_Array(self, mac, arr):
mac_bin = mac.encode("ascii");
mac_len = len(mac_bin);
arr_bin = array.array('d', arr).tobytes();
arr_len = len(arr_bin);
m = struct.pack('qqd%ss%ss' % (mac_len, arr_len), mac_len, arr_len, time.time(), mac_bin, arr_bin);
return m
Here are the docs for python's struct (refer to section 7.3.2.2. Format Characters):
https://docs.python.org/2/library/struct.html
1) The issue is what does 'qqd%ss%ss' mean ???
Does it mean -> long,long,double,char,char,[],char[],char,char[],char[]
2) why is modulo "%" used here with a tuple 'qqd%ss%ss' % (mac_len, arr_len) ?
The first argument to pack is the result of the expression 'qqd%ss%ss' % (mac_len, arr_len), where the two %s are replaced by the values of the given variables. Assuming mac_len == 8 and arr_len == 4, for example, the result is qqd8s4s. s preceded by a number simply means to copy the given bytes for that format into the result.
I was trying to build this bytes object in Python 3:
b'3\r\n'
so I tried the obvious (for me), and found a weird behaviour:
>>> bytes(3) + b'\r\n'
b'\x00\x00\x00\r\n'
Apparently:
>>> bytes(10)
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
I've been unable to see any pointers on why the bytes conversion works this way reading the documentation. However, I did find some surprise messages in this Python issue about adding format to bytes (see also Python 3 bytes formatting):
http://bugs.python.org/issue3982
This interacts even more poorly with oddities like bytes(int) returning zeroes now
and:
It would be much more convenient for me if bytes(int) returned the ASCIIfication of that int; but honestly, even an error would be better than this behavior. (If I wanted this behavior - which I never have - I'd rather it be a classmethod, invoked like "bytes.zeroes(n)".)
Can someone explain me where this behaviour comes from?
From python 3.2 you can use to_bytes:
>>> (1024).to_bytes(2, byteorder='big')
b'\x04\x00'
def int_to_bytes(x: int) -> bytes:
return x.to_bytes((x.bit_length() + 7) // 8, 'big')
def int_from_bytes(xbytes: bytes) -> int:
return int.from_bytes(xbytes, 'big')
Accordingly, x == int_from_bytes(int_to_bytes(x)).
Note that the above encoding works only for unsigned (non-negative) integers.
For signed integers, the bit length is a bit more tricky to calculate:
def int_to_bytes(number: int) -> bytes:
return number.to_bytes(length=(8 + (number + (number < 0)).bit_length()) // 8, byteorder='big', signed=True)
def int_from_bytes(binary_data: bytes) -> Optional[int]:
return int.from_bytes(binary_data, byteorder='big', signed=True)
That's the way it was designed - and it makes sense because usually, you would call bytes on an iterable instead of a single integer:
>>> bytes([3])
b'\x03'
The docs state this, as well as the docstring for bytes:
>>> help(bytes)
...
bytes(int) -> bytes object of size given by the parameter initialized with null bytes
You can use the struct's pack:
In [11]: struct.pack(">I", 1)
Out[11]: '\x00\x00\x00\x01'
The ">" is the byte-order (big-endian) and the "I" is the format character. So you can be specific if you want to do something else:
In [12]: struct.pack("<H", 1)
Out[12]: '\x01\x00'
In [13]: struct.pack("B", 1)
Out[13]: '\x01'
This works the same on both python 2 and python 3.
Note: the inverse operation (bytes to int) can be done with unpack.
Python 3.5+ introduces %-interpolation (printf-style formatting) for bytes:
>>> b'%d\r\n' % 3
b'3\r\n'
See PEP 0461 -- Adding % formatting to bytes and bytearray.
On earlier versions, you could use str and .encode('ascii') the result:
>>> s = '%d\r\n' % 3
>>> s.encode('ascii')
b'3\r\n'
Note: It is different from what int.to_bytes produces:
>>> n = 3
>>> n.to_bytes((n.bit_length() + 7) // 8, 'big') or b'\0'
b'\x03'
>>> b'3' == b'\x33' != b'\x03'
True
The documentation says:
bytes(int) -> bytes object of size given by the parameter
initialized with null bytes
The sequence:
b'3\r\n'
It is the character '3' (decimal 51) the character '\r' (13) and '\n' (10).
Therefore, the way would treat it as such, for example:
>>> bytes([51, 13, 10])
b'3\r\n'
>>> bytes('3', 'utf8') + b'\r\n'
b'3\r\n'
>>> n = 3
>>> bytes(str(n), 'ascii') + b'\r\n'
b'3\r\n'
Tested on IPython 1.1.0 & Python 3.2.3
The ASCIIfication of 3 is "\x33" not "\x03"!
That is what python does for str(3) but it would be totally wrong for bytes, as they should be considered arrays of binary data and not be abused as strings.
The most easy way to achieve what you want is bytes((3,)), which is better than bytes([3]) because initializing a list is much more expensive, so never use lists when you can use tuples. You can convert bigger integers by using int.to_bytes(3, "little").
Initializing bytes with a given length makes sense and is the most useful, as they are often used to create some type of buffer for which you need some memory of given size allocated. I often use this when initializing arrays or expanding some file by writing zeros to it.
I was curious about performance of various methods for a single int in the range [0, 255], so I decided to do some timing tests.
Based on the timings below, and from the general trend I observed from trying many different values and configurations, struct.pack seems to be the fastest, followed by int.to_bytes, bytes, and with str.encode (unsurprisingly) being the slowest. Note that the results show some more variation than is represented, and int.to_bytes and bytes sometimes switched speed ranking during testing, but struct.pack is clearly the fastest.
Results in CPython 3.7 on Windows:
Testing with 63:
bytes_: 100000 loops, best of 5: 3.3 usec per loop
to_bytes: 100000 loops, best of 5: 2.72 usec per loop
struct_pack: 100000 loops, best of 5: 2.32 usec per loop
chr_encode: 50000 loops, best of 5: 3.66 usec per loop
Test module (named int_to_byte.py):
"""Functions for converting a single int to a bytes object with that int's value."""
import random
import shlex
import struct
import timeit
def bytes_(i):
"""From Tim Pietzcker's answer:
https://stackoverflow.com/a/21017834/8117067
"""
return bytes([i])
def to_bytes(i):
"""From brunsgaard's answer:
https://stackoverflow.com/a/30375198/8117067
"""
return i.to_bytes(1, byteorder='big')
def struct_pack(i):
"""From Andy Hayden's answer:
https://stackoverflow.com/a/26920966/8117067
"""
return struct.pack('B', i)
# Originally, jfs's answer was considered for testing,
# but the result is not identical to the other methods
# https://stackoverflow.com/a/31761722/8117067
def chr_encode(i):
"""Another method, from Quuxplusone's answer here:
https://codereview.stackexchange.com/a/210789/140921
Similar to g10guang's answer:
https://stackoverflow.com/a/51558790/8117067
"""
return chr(i).encode('latin1')
converters = [bytes_, to_bytes, struct_pack, chr_encode]
def one_byte_equality_test():
"""Test that results are identical for ints in the range [0, 255]."""
for i in range(256):
results = [c(i) for c in converters]
# Test that all results are equal
start = results[0]
if any(start != b for b in results):
raise ValueError(results)
def timing_tests(value=None):
"""Test each of the functions with a random int."""
if value is None:
# random.randint takes more time than int to byte conversion
# so it can't be a part of the timeit call
value = random.randint(0, 255)
print(f'Testing with {value}:')
for c in converters:
print(f'{c.__name__}: ', end='')
# Uses technique borrowed from https://stackoverflow.com/q/19062202/8117067
timeit.main(args=shlex.split(
f"-s 'from int_to_byte import {c.__name__}; value = {value}' " +
f"'{c.__name__}(value)'"
))
The behaviour comes from the fact that in Python prior to version 3 bytes was just an alias for str. In Python3.x bytes is an immutable version of bytearray - completely new type, not backwards compatible.
From bytes docs:
Accordingly, constructor arguments are interpreted as for bytearray().
Then, from bytearray docs:
The optional source parameter can be used to initialize the array in a few different ways:
If it is an integer, the array will have that size and will be initialized with null bytes.
Note, that differs from 2.x (where x >= 6) behavior, where bytes is simply str:
>>> bytes is str
True
PEP 3112:
The 2.6 str differs from 3.0’s bytes type in various ways; most notably, the constructor is completely different.
int (including Python2's long) can be converted to bytes using following function:
import codecs
def int2bytes(i):
hex_value = '{0:x}'.format(i)
# make length of hex_value a multiple of two
hex_value = '0' * (len(hex_value) % 2) + hex_value
return codecs.decode(hex_value, 'hex_codec')
The reverse conversion can be done by another one:
import codecs
import six # should be installed via 'pip install six'
long = six.integer_types[-1]
def bytes2int(b):
return long(codecs.encode(b, 'hex_codec'), 16)
Both functions work on both Python2 and Python3.
Although the prior answer by brunsgaard is an efficient encoding, it works only for unsigned integers. This one builds upon it to work for both signed and unsigned integers.
def int_to_bytes(i: int, *, signed: bool = False) -> bytes:
length = ((i + ((i * signed) < 0)).bit_length() + 7 + signed) // 8
return i.to_bytes(length, byteorder='big', signed=signed)
def bytes_to_int(b: bytes, *, signed: bool = False) -> int:
return int.from_bytes(b, byteorder='big', signed=signed)
# Test unsigned:
for i in range(1025):
assert i == bytes_to_int(int_to_bytes(i))
# Test signed:
for i in range(-1024, 1025):
assert i == bytes_to_int(int_to_bytes(i, signed=True), signed=True)
For the encoder, (i + ((i * signed) < 0)).bit_length() is used instead of just i.bit_length() because the latter leads to an inefficient encoding of -128, -32768, etc.
Credit: CervEd for fixing a minor inefficiency.
As you want to deal with binary representation, the best is to use ctypes.
import ctypes
x = ctypes.c_int(1234)
bytes(x)
You must use the specific integer representation (signed/unsigned and the number of bits: c_uint8, c_int8, c_unit16,...).
Some answers don't work with large numbers.
Convert integer to the hex representation, then convert it to bytes:
def int_to_bytes(number):
hrepr = hex(number).replace('0x', '')
if len(hrepr) % 2 == 1:
hrepr = '0' + hrepr
return bytes.fromhex(hrepr)
Result:
>>> int_to_bytes(2**256 - 1)
b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
I think you can convert the int to str first, before you convert to byte.
That should produce the format you want.
bytes(str(your_number),'UTF-8') + b'\r\n'
It works for me in py3.8.
If the question is how to convert an integer itself (not its string equivalent) into bytes, I think the robust answer is:
>>> i = 5
>>> i.to_bytes(2, 'big')
b'\x00\x05'
>>> int.from_bytes(i.to_bytes(2, 'big'), byteorder='big')
5
More information on these methods here:
https://docs.python.org/3.8/library/stdtypes.html#int.to_bytes
https://docs.python.org/3.8/library/stdtypes.html#int.from_bytes
>>> chr(116).encode()
b't'
I am trying to convert the following code from c to Python. The C code looks like:
seed = (time(0) ^ (getpid() << 16));
fprintf("0x%08x \n", seed);
that outputs values like 0x7d24defb.
And the python code:
time1 = int(time.time())
seed = (time1 ^ (os.getpid() <<16))
that outputs values like: 1492460964
What do i need to modify at the python code so I get address-like values?
It depends on the way the value is displayed. The %x flag in printf-functions displays the given value in hexadecimal. In Python you can use the hex function to convert the value to a hexadecimal representation.
The equivalent Python code to: fprintf("0x%08x \n", seed);
>>> '0x{:08x}"'.format(1492460964)
'0x58f525a4"'
Note that hex() alone won't pad zeros to size 8 like the C code does.
I suppose this is what you what:
>>> n =hex (int(time.time()) ^ (os.getpid() <<16))
>>> print n
0x431c2fd2
>>>