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Converting integer to 8-bit ASCII characters, NOT Unicode in Python 3

I've been working on a project where I'm encoding numbers as characters. Being used to C++, I assumed I could just use any 8bit number and cast it to a character. However, python's chr() function is returning Unicode characters, which aren't 8-bit, so that will not work.
I am new to Python and, from what I've read, previous versions used to have 2 separate functions: chr() for ASCII characters and unichr() for Unicode characters.
I am also limited to what I can get in the standard python library for windows (we are not allowed to install modules with pip).
This might usually be okay, but here's an example of when this can mess with my program:
If I'm encoding the integer 143:
# this is not taken from my actual code
num = 143
c = chr(143)
print(c)
I would expect this to print the ASCII character (a capital A with a little circle above it). Instead, I get the unicode \x8f, which represents "SS3" (Single Shift 3).
TL;DR: I'm converting 8-bit numbers to characters, but chr() converts to Unicode and I REALLY need a way to convert to ASCII instead, but I can't seem to find it in the standard library.
I know that this is such a simple problem and it's extremely frustrating to be stuck on this of all things.
Thanks a lot in advance!
Have a nice day!
- Vlad

"A with a little circle above it" is not an ASCII character, and 143 is outside the ASCII range (0-127).
It seems you are thinking in terms of the encoded bytes rather than unicode codepoints (which Python3 uses to represent string values). See here for 8 bit encodings where b'\x8f' represents 'Å‎'.
You probably want to do something like this:
import sys
c = 143
# Convert to byte
b = c.to_bytes(1, sys.byteorder)
# Decode to unicode (str) and print
print(b.decode('cp437'))
Å‎
You could also take a look at the struct package in the standard library, which deals with bytes and chars in a more "C-like" fashion.

Python: Converting special characters into operable integers?

I am currently working on a really simple encryption project algorithm to show basic understanding of how encryption works, and my encryption algorithm basically just uses the 'ord()' function for converting standard ASCII characters into integers that the algorithm can work on.
The problem I have run into is that I also need my program to be capable of encrypting, for example, the contents of a Windows executable (EXE) file. To do so, I need to convert all sorts of special characters (Not ASCII) into integers that I can operate off of.
I don't know a whole lot about encoding, but from what I understand, 'ord()' only works because there is a ASCII character map that has a corresponding number for each character. I couldn't seem to figure how to convert the special characters of an EXE file straight to integers, so I tried converting to bytes which seems a little more universal to me (please correct me if I am wrong).
At this point, I am just looking for a solution to be able to read an EXE file, and convert each character into a number specific to that character (for encryption/ decryption purposes).

You are confusing the meaning assigned to bytes (like the ASCII standard) with the bytes themselves. ord() just gives you the numerical value for a given byte. That Python interprets those bytes and shows you ASCII codepoints is neither here nor there.
In other words, ord() doesn't have to consult an ASCII table and can handle any byte value. All it has to do is take the already known byte value and give you a Python int object for it.
Read your data as binary (open the file with b added to the file mode), and use ord(). In Python 2, that'll result in str objects, and each character in such an object is really a byte value in the range 0 - 255.
Note that if you are using Python 3, reading from a file in binary mode results in a bytes object that makes it clearer still that these are integer values in a range:
>>> b'abc'
b'abc'
>>> b'abc'[0]
97
Indexing to an individual point in a bytes object produces the integer value and no call to ord() is required.

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.

Confused about unicode representations

I am confused about hex representation of Unicode.
I have an example file with a single mathematical integral sign character in it. That is U+222B
If I cat the file or edit it in vi I get an integral sign displayed.
A hex dump of the file shows its hex content is 88e2 0aab
In python I can create an integral unicode character and print p rendering on my terminal and integral sign.
>>> p=u'\u222b'
>>> p
u'\u222b'
>>> print p
∫
What confuses me is I can open a file with the integral sign in it, get the integral symbol but the hex content is different.
>>> c=open('mycharfile','r').read()
>>> c
'\xe2\x88\xab\n'
>>> print c
∫
One is a Unicode object and one is a plain string but what is the relationship between the two hex codes apparently for the same character? How would I manually convert one to another?

The plain string has been encoded using UTF-8, one of a variety of ways to represent Unicode code points in bytes. UTF-8 is a multibyte encoding which has the often useful feature that it is a superset of ASCII - the same byte encodes any ASCII character in UTF-8 or in ASCII.
In Python 2.x, use the encode method on a Unicode object to encode it, and decode or the unicode constructor to decode it:
>>> u'\u222b'.encode('utf8')
'\xe2\x88\xab'
>>> '\xe2\x88\xab'.decode('utf8')
u'\u222b'
>>> unicode('\xe2\x88\xab', 'utf8')
u'\u222b'
print, when given a Unicode argument, implicitly encodes it. On my system:
>>> sys.stdout.encoding
'UTF-8'
See this answer for a longer discussion of print's behavior:
Why does Python print unicode characters when the default encoding is ASCII?
Python 3 handles things a bit differently; the changes are documented here:
http://docs.python.org/3.0/whatsnew/3.0.html#text-vs-data-instead-of-unicode-vs-8-bit

Okay i have it. Thanks for the answers. i wanted to see how to do the conversion rather than convert a string using Python.
the conversion works this way.
If you have a unicode character, in my example an integral symbol.
Octal dump produces
echo -n "∫"|od -x
0000000 88e2 00ab
Each hex pair are reversed so it really means
e288ab00
The first hex character is E. the high bit means this is a Unicode string and the next two bits indicate it is 3 three bytes (16 bits) to represent the character.
The first two bits of the remaining hex digits are throw away (they signify they are unicode.) the full bit stream is
111000101000100010101011
Throw away the first 4 bits and the first two bits of the remaining hex digits
0010001000101011
Re-expressing this in hex
222B
They you have it!

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') ?