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Can't regex string between string and line break - str(x) versus x.decode()

Why does a regex fail of a string cast from an object when line breaks are present?
That is why does this fail to find a match (ie print 'Green') in a string created from str(obj):
import re
s = str(b'Package Name: Green\\r\\n Release version: 8.1\\r\\n')
match = re.search(r'Package Name: (.*)\r\n', s)
print(match.group(1))
When this succeeds using a string created from obj.decode()?
import re
s = (b'Package Name: Green\\r\\n Release version: 8.1\\r\\n').decode()
match = re.search(r'Package Name: (.*)\r\n', s)
print(match.group(1))
No matter what search pattern was tried, searching the string created by str(obj) failed to find a match...

The reason you get different results is that you’re doing different things. Calling str on a bytes with newline characters returns a string with a literal backslash and n; calling decode returns a string with a newline character in it. So, if you’re searching the results for newline characters, the second one will succeed, and the first will fail. And it’s the second one that you wanted.
In other words, using decode here is right, and str is wrong; that’s why you get different results. If you can’t think through the difference, try just printing them out: print(b.decode()); print(str(b)) and you’ll see the difference immediately.
In fact, you should usually be decoding the strings as soon as you receive them, and never looking at the bytes again. Then you never have to worry about the str representation of bytes objects (except maybe in some code that logs errors caused by invalid strings that you couldn’t decode). The only exception is when you know the bytes are some kind of encoded text, but can’t be sure what the encoding is. For example, if you’re parsing HTTP headers or email messages or Python source code, you don’t know the character set until you read part of the file and search it for special ASCII-encoded strings. Or, if you’re converting a bunch of old text files from Windows to Unix line endings and some are cp1252 while others are cp1250, you don’t care which is which because they both encode line endings the same way. For those cases, just stick with bytes, and search for b'\n' instead of '\n'.
If you want to know why Python makes this so complicated:
bytes objects are used to store strings encoded in your default encoding—but they’re also used to store strings encoded in different encodings, and binary data that isn’t a string at all. And a bytes object has no idea which of those it’s storing; they’re all just sequences of numbers.
Python 2 effectively assumed that a bytes was being used to store a string in your default encoding, so it let you convert back and forth to Unicode by calling functions like str, or even concatenating a bytes and Unicode string. That turned out to be one of the biggest sources of errors in the language. You still see Python 2 users posting questions here every few days asking why they got a UnicodeEncodeError when they weren’t calling encode anywhere (or, worse, when they were calling decode), and fixing that was one of the main reasons for Python 3’s existence.
The human-readable representation of a bytes object has to be something that can be produced without error, and read unambiguously, whether it’s a string in the default encoding, a string in a completely different encoding, or a sequence of pixel brightness values ranging from 0 to 255. The compromise solution (for things like that HTTP headers case above) is the backslash-escaped quoted string.
By the way, during the Python 2 to 3 transition, the core devs assumed multiple people would come up with clever EncodedBytes types that carried around their encoding, and could therefore act more like Python 2 byte strings but without all the associated errors, and after a couple years one of them would be the clear winner on PyPI and maybe they could add it to Python 3.3 or so. That’s what you’re probably instinctively reaching for here. But, as it turned out, nobody used any such libraries, because it’s almost always easier to just decode and encode at the edges of your program and use Unicode everywhere, and the exceptions are almost always cases where you don’t know the encoding so EncodedBytes wouldn’t help.
One last thing: thinking of functions like str or float as “casts” is misleading. While it looks superficially similar to the way you do explicit casts in C or Java or Go or whatever language you’re used to, it has a very different meaning

What is actually happening when I encode a string into bytes? [duplicate]

What's a Python bytestring?
All I can find are topics on how to encode to bytestring or decode to ASCII or UTF-8. I'm trying to understand how it works under the hood. In a normal ASCII string, it's an array or list of characters, and each character represents an ASCII value from 0-255, so that's how you know what character is represented by the number. In Unicode, it's the 8- or 16-byte representation for the character that tells you what character it is.
So what is a bytestring? How does Python know which characters to represent as what? How does it work under the hood? Since you can print or even return these strings and it shows you the string representation, I don't quite get it...
Ok, so my point is definitely getting missed here. I've been told that it's an immutable sequence of bytes without any particular interpretation.
A sequence of bytes.. Okay, let's say one byte:
'a'.encode() returns b'a'.
Simple enough. Why can I read the a?
Say I get the ASCII value for a, by doing this:
printf "%d" "'a"
It returns 97. Okay, good, the integer value for the ASCII character a. If we interpret 97 as ASCII, say in a C char, then we get the letter a. Fair enough. If we convert the byte representation to bits, we get this:
01100001
2^0 + 2^5 + 2^6 = 97. Cool.
So why is 'a'.encode() returning b'a' instead of 01100001??
If it's without a particular interpretation, shouldn't it be returning something like b'01100001'?
It seems like it's interpreting it like ASCII.
Someone mentioned that it's calling __repr__ on the bytestring, so it's displayed in human-readable form. However, even if I do something like:
with open('testbytestring.txt', 'wb') as f:
f.write(b'helloworld')
It will still insert helloworld as a regular string into the file, not as a sequence of bytes... So is a bytestring in ASCII?

It is a common misconception that text is ASCII or UTF-8 or Windows-1252, and therefore bytes are text.
Text is only text, in the way that images are only images. The matter of storing text or images to disk is a matter of encoding that data into a sequence of bytes. There are many ways to encode images into bytes: JPEG, PNG, SVG, and likewise many ways to encode text, ASCII, UTF-8 or Windows-1252.
Once encoding has happened, bytes are just bytes. Bytes are not images anymore; they have forgotten the colors they mean; although an image format decoder can recover that information. Bytes have similarly forgotten the letters they used to be. In fact, bytes don't remember whether they were images or text at all. Only out of band knowledge (filename, media headers, etcetera) can guess what those bytes should mean, and even that can be wrong (in case of data corruption).
so, in Python (Python 3), we have two types for things that might otherwise look similar; For text, we have str, which knows it's text; it knows which letters it's supposed to mean. It doesn't know which bytes that might be, since letters are not bytes. We also have bytestring, which doesn't know if it's text or images or any other kind of data.
The two types are superficially similar, since they are both sequences of things, but the things that they are sequences of is quite different.
Implementationally, str is stored in memory as UCS-? where the ? is implementation defined, it may be UCS-4, UCS-2 or UCS-1, depending on compile time options and which code points are present in the represented string.
"But why"?
Some things that look like text are actually defined in other terms. A really good example of this are the many Internet protocols of the world. For instance, HTTP is a "text" protocol that is in fact defined using the ABNF syntax common in RFCs. These protocols are expressed in terms of octets, not characters, although an informal encoding may also be suggested:
2.3. Terminal Values
Rules resolve into a string of terminal values, sometimes called
characters. In ABNF, a character is merely a non-negative integer.
In certain contexts, a specific mapping (encoding) of values into a
character set (such as ASCII) will be specified.
This distinction is important, because it's not possible to send text over the internet, the only thing you can do is send bytes. saying "text but in 'foo' encoding" makes the format that much more complex, since clients and servers need to now somehow figure out the encoding business on their own, hopefully in the same way, since they must ultimately pass data around as bytes anyway. This is doubly useless since these protocols are seldom about text handling anyway, and is only a convenience for implementers. Neither the server owners nor end users are ever interested in reading the words Transfer-Encoding: chunked, so long as both the server and the browser understand it correctly.
By comparison, when working with text, you don't really care how it's encoded. You can express the "Heävy Mëtal Ümlaüts" any way you like, except "Heδvy Mλtal άmlaόts"
The distinct types thus give you a way to say "this value 'means' text" or "bytes".

Python does not know how to represent a bytestring. That's the point.
When you output a character with value 97 into pretty much any output window, you'll get the character 'a' but that's not part of the implementation; it's just a thing that happens to be locally true. If you want an encoding, you don't use bytestring. If you use bytestring, you don't have an encoding.
Your piece about .txt files shows you have misunderstood what is happening. You see, plain text files too don't have an encoding. They're just a series of bytes. These bytes get translated into letters by the text editor but there is no guarantee at all that someone else opening your file will see the same thing as you if you stray outside the common set of ASCII characters.

As the name implies, a Python 3 bytestring (or simply a str in Python 2.7) is a string of bytes. And, as others have pointed out, it is immutable.
It is distinct from a Python 3 str (or, more descriptively, a unicode in Python 2.7) which is a
string of abstract Unicode characters (a.k.a. UTF-32, though Python 3 adds fancy compression under the hood to reduce the actual memory footprint similar to UTF-8, perhaps even in a more general way).
There are essentially three ways of "interpreting" these bytes. You can look at the numeric value of an element, like this:
>>> ord(b'Hello'[0]) # Python 2.7 str
72
>>> b'Hello'[0] # Python 3 bytestring
72
Or you can tell Python to emit one or more elements to the terminal (or a file, device, socket, etc.) as 8-bit characters, like this:
>>> print b'Hello'[0] # Python 2.7 str
H
>>> import sys
>>> sys.stdout.buffer.write(b'Hello'[0:1]) and None; print() # Python 3 bytestring
H
As Jack hinted at, in this latter case it is your terminal interpreting the character, not Python.
Finally, as you have seen in your own research, you can also get Python to interpret a bytestring. For example, you can construct an abstract unicode object like this in Python 2.7:
>>> u1234 = unicode(b'\xe1\x88\xb4', 'utf-8')
>>> print u1234.encode('utf-8') # if terminal supports UTF-8
ሴ
>>> u1234
u'\u1234'
>>> print ('%04x' % ord(u1234))
1234
>>> type(u1234)
<type 'unicode'>
>>> len(u1234)
1
>>>
Or like this in Python 3:
>>> u1234 = str(b'\xe1\x88\xb4', 'utf-8')
>>> print (u1234) # if terminal supports UTF-8 AND python auto-infers
ሴ
>>> u1234.encode('unicode-escape')
b'\\u1234'
>>> print ('%04x' % ord(u1234))
1234
>>> type(u1234)
<class 'str'>
>>> len(u1234)
1
(and I am sure that the amount of syntax churn between Python 2.7 and Python3 around bystestring, strings, and Unicode had something to do with the continued popularity of Python 2.7. I suppose that when Python 3 was invented they didn't yet realize that everything would become UTF-8 and therefore all the fuss about abstraction was unnecessary).
But the Unicode abstraction does not happen automatically if you don't want it to. The point of a bytestring is that you can directly get at the bytes. Even if your string happens to be a UTF-8 sequence, you can still access bytes in the sequence:
>>> len(b'\xe1\x88\xb4')
3
>>> b'\xe1\x88\xb4'[0]
'\xe1'
And this works in both Python 2.7 and Python 3, with the difference being that in Python 2.7 you have str, while in Python3 you have bytestring.
You can also do other wonderful things with bytestrings, like knowing if they will fit in a reserved space within a file, sending them directly over a socket, calculating the HTTP content-length field correctly, and avoiding Python Bug 8260. In short, use bytestrings when your data is processed and stored in bytes.

Bytes objects are immutable sequences of single bytes. The documentation has a very good explanation of what they are and how to use them.

python proper way to decode raw udp packet

I'm making a script to get Valve's server information (players online, map, etc)
the packet I get when I request for information is this:
'\xff\xff\xff\xffI\x11Stargate Central CAP SBEP\x00sb_wuwgalaxy_fix\x00garrysmod\x00Spacebuild\x00\xa0\x0f\n\x0c\x00dw\x00\x0114.09.08\x00\xb1\x87i\x06\xb4g\x17.\x15#\x01gm:spacebuild3\x00\xa0\x0f\x00\x00\x00\x00\x00\x00'
This may help you to see what I'm trying to do https://developer.valvesoftware.com/wiki/Server_queries#A2S_INFO
The problem is, I don't know how to decode this properly, it's easy to get the string but I have no idea how to get other types like byte and short
for example '\xa0\x0f'
For now I'm doing multiple split but do you know if there is any better way of doing this?

Python has functions for encoding/decoding different data types into bytes. Take a look at the struct package, the functions struct.pack() and struct.unpack() are your friends there.
taken from https://docs.python.org/2/library/struct.html
>>> from struct import *
>>> pack('hhl', 1, 2, 3)
'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('hhl', '\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
The first argument of the unpack function defines the format of the data stored in the second argument. Now you need to translate the description given by valve to a format string. If you wanted to unpack 2 bytes and a short from a data string (that would have a length of 4 bytes, of course), you could do something like this:
(first_byte, second_byte, the_short) = unpack("cc!h", data)
You'll have to take care yourself, to get the correct part of the data string (and I don't know if those numbers are signed or not, be sure to take care of that, too).
The strings you'll have to do differently (they are null-terminated here, so start were you know a string starts and read to the first "\0" byte).
pack() work's the other way around and stores data in a byte string. Take a look at the examples on the python doc and play around with it a bit to get a feel for it (when a tuple is returned/needed, e.g.).
struct supports you in getting the right byte order, which most of the time is network byte order and different from your system. That is of course only necessary for multi byte integers (like short) - so a format string of `"!h" should unpack a short correctly.

Serialization via pickle/eval and zlib

I wish to compress a string using zlib and append it to a text message as a string. I have a couple of issues with it:
a. Is there a problem to combine "binary" string with normal string? For example, is there any problem sending via socket a string that looks like that:
MSG 10=12 20=x\x9c+(\xc0\x00\x00S3\x08Q 33=hansz
I ask it since when opening files one usually declares whether he intends to read at binary mode or not, and I never fully understood that.
b. Can I be sure that some characters will not appear in the compressed string? For example, if the compressed string will include some char sequence like x\x9c 33=eve, I'll have trouble parsing the message properly. If I know that whitespaces will never appear in a zlib compressed string, I can do some string split; If I know that quotes and apostrophes do not appear, I might use shlex split.
c. My intention is to use either zlib.compress(str(obj)) or zlib.compress(pickle.dumps(obj)) as kind of pickling, and use either eval(zlib.decompress(s)) or pickle.loads(zlib.decompress(s)) for unpickling. Do you think it makes sense? The first idea is less safe (as eval is never that safe), but it's an inner system, so I'm ok with it, and on the other hand the compressing turns out to be shorter on most cases, and as quick. Do you think it's a good practice?
d. The reason I wish to have these messages short is that I wish to send them later via socket. I am not proficient with sockets, however, I know these tend to read small (4k?) buffers, so I try to make my messages not much longer than that.

a. The Problem with combining bytes and a unicode string is the following: There are more letters that 255. So historically, hundreds of encodings were created to put different alphabets into one byte.
>>> print b'\xE4'.decode('cp1251') # russian d
д
>>> print b'\xE4'.decode('cp1252') # german ae
ä
The letters have different meaning. To not loose the meaning of these letters, you use unicode.
>>> print u'\u00e4\u0434'
äд
However when you see bytes then you may not know the encoding. So you can not combine unicode and bytes straight away because one byte may be different letters.
Use 'UTF-8' as encoding for the next years. It uses more than one byte if necessairy and stores all letters.
b. zlib takes bytes and outputs bytes. It can contain any byte.
c. zlib.compress(pickle.dumps(obj)) and pickle.loads(zlib.decompress(s)) is totally fine. Pickle takes objects and returns bytes. You can save and store more objects than with zlib.compress(repr(obj)) and eval(zlib.decompress(s)). pickle is as safe as eval. If you need save evaluation have a look at import ast ast.literal_eval or use json instead of pickle.
d. Make sure to know when a message ends and another message starts. I think you can use zlib.decompressobj for this. Otherwise zib can get confused. Sockets can send much more that 4k bytes. The buffer means that a socket saves up to 4k bytes and does not want to receive more until you take bytes out of the buffer. If you use TCP you can send endless streams of bytes and nothing is lost.

Decode base64 string in python 3 (with lxml or not)

I know this looks embarrassingly easy, and I guess the problem is that I just don't have a clear understanding of all this bytes-str-unicode (and encoding-decoding, speaking frankly) stuff yet.
I've been trying to get my working code to run on Python 3. The part I'm stuck with is when I parse an XML with lxml and decode a base64 string that is in that XML.
The code now works in the following manner:
I retrieve the binary data with an XPath query '.../binary/text()'. This produces a one-element list containing a lxml.etree._ElementUnicodeResult object. Then, with python 2, I was able to do:
decoded = source.decode('base64')
and finally
output = numpy.frombuffer(decoded)
However, on python 3 I get an error message saying
AttributeError: 'lxml.etree._ElementUnicodeResult' object has no attribute 'decode'
This is not so surprising, because lxml.etree._ElementUnicodeResult is a subclass of str.
Another way would be to get a real str with the same data in it with
binary = tree.xpath('//binary')[0]
binary_string = binary.text
That would be essentially the same. So what do I do to decode it from base64? I've looked at the base64 module, but it takes a bytes object as an argument, and I can't think of the way to present str as bytes, because if I try to construct a bytes object, Python will try to encode the string, which I don't need.
Googling further, I came across the binascii module (which is invoked indirectly from base64 anyway, if I'm not mistaken), but calling binascii.b2a_base64() on my string produces
TypeError: 'str' does not support the buffer interface
P.S. I've even found an answered question on how to decode a hex string in Python 3, but this is done with a dedicated method bytes.fromhex() so I don't see how it would be helpful.
Could someone please tell me what I'm missing? I'm afraid most of the post is irrelevant and only aggravates my shame, but at least you guys know what I tried.

OK, I think I'm going to summarize my current understanding of things (feel free to correct me). Hopefully it will help someone else out there as confused as I've been.
The credit totally goes to thebjorn and delnan, of course.
So, starting with the most common things:
there's Unicode, and it's a global standard that assigns codes (or code points) to all the exotic characters you can imagine. Those codes are just integer numbers. As of Unicode 6.1 there are 109,975 graphic characters, says Wikipedia.
Then there are encodings that define how to designate Unicode characters with byte codes. One byte isn't enough to designate an arbitrary Unicode char. Although, if you only take a small subset of them (English alphabet, digits, punctuation, some control characters), you can do with one byte per character (or even 7 bits; see ASCII).
To pass a Unicode string anywhere, one needs to encode it in bytes, then it can be decoded on the other end.
In Python 2, str is actually bytes, and unicode is Unicode, but Python 2 will do implicit encoding/decoding for you when needed. It will try to use ASCII encoding.
In Python 3, str is always a Unicode string, and bytes is a new data type for actual bytes. No implicit conversion is ever done by Python 3, you always need to do it yourself and specify the encoding. That means that your program won't work until you understand what's going on, which totally happened to me.
Now, that being more or less clear, let's move on to base64 encoding, which is also an encoding of sorts, but has a slightly different meaning.
Suppose you have some binary data (i.e. bytes) that may mean anything (in my case it's a bunch of floats). Now you want to represent this binary array with a string. That's what base64 encoding means: you have your bytes represented as an ASCII string.
Base64 means 6 bit, so in a base64-encoded string a single character stands for 6 bits of your data. That is why base64-encoded strings need to have the length that is a multiple of 4: otherwise the number of bytes encoded will be not integer.
Finally, to decode from base64 you need an ASCII string. A Unicode string won't do, there can only be characters from the base64 alphabet. Base64 module does the job in Python. The base64.b64decode() function takes a byte string as the argument. In Python 2 it means: str. In Python 3 it means: bytes. So if you have a str, such as
>>> s = 'U3RhY2sgT3ZlcmZsb3c='
In Python 2 you could just do
>>> s.decode('base64')
because s is already in ASCII.
In Python 3, you need to encode it in ASCII first, so you'll have to do:
>>> base64.b64decode(s.encode('ascii'))
And by the way, this will return a bytes object, so it's really up to you how to treat those bytes then. Maybe it's my floats, but maybe you should try to decode it as ASCII :)
In Python 2 however it will be just a str. Anyway, have a look at struct for the tools to unpack your data from those bytes.
So if you need the code to work on both Python 2 and 3, go with the last one. To make sure you have Unicode in the end (if you are decoding text from base64), you'll have to decode it:
>>> base64.b64decode(s.encode('ascii')).decode('ascii')
On Python 2, encode('ascii') won't effectively do anything because it's applied to str. So it will do an implicit conversion to Unicode first, and then do what you want (convert it back to ASCII). decode('ascii') will return a unicode object on Python 2.

I don't have Python 3 installed, but it sounds like you need to convert the Unicode returned from lxml to bytes, perhaps by calling .encode('ascii') ?