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In 'Automating Boring Stuff Using Python' Page 208, I cannot understand this line of code [duplicate]

While asking this question, I realized I didn't know much about raw strings. For somebody claiming to be a Django trainer, this sucks.
I know what an encoding is, and I know what u'' alone does since I get what is Unicode.
But what does r'' do exactly? What kind of string does it result in?
And above all, what the heck does ur'' do?
Finally, is there any reliable way to go back from a Unicode string to a simple raw string?
Ah, and by the way, if your system and your text editor charset are set to UTF-8, does u'' actually do anything?

There's not really any "raw string"; there are raw string literals, which are exactly the string literals marked by an 'r' before the opening quote.
A "raw string literal" is a slightly different syntax for a string literal, in which a backslash, \, is taken as meaning "just a backslash" (except when it comes right before a quote that would otherwise terminate the literal) -- no "escape sequences" to represent newlines, tabs, backspaces, form-feeds, and so on. In normal string literals, each backslash must be doubled up to avoid being taken as the start of an escape sequence.
This syntax variant exists mostly because the syntax of regular expression patterns is heavy with backslashes (but never at the end, so the "except" clause above doesn't matter) and it looks a bit better when you avoid doubling up each of them -- that's all. It also gained some popularity to express native Windows file paths (with backslashes instead of regular slashes like on other platforms), but that's very rarely needed (since normal slashes mostly work fine on Windows too) and imperfect (due to the "except" clause above).
r'...' is a byte string (in Python 2.*), ur'...' is a Unicode string (again, in Python 2.*), and any of the other three kinds of quoting also produces exactly the same types of strings (so for example r'...', r'''...''', r"...", r"""...""" are all byte strings, and so on).
Not sure what you mean by "going back" - there is no intrinsically back and forward directions, because there's no raw string type, it's just an alternative syntax to express perfectly normal string objects, byte or unicode as they may be.
And yes, in Python 2.*, u'...' is of course always distinct from just '...' -- the former is a unicode string, the latter is a byte string. What encoding the literal might be expressed in is a completely orthogonal issue.
E.g., consider (Python 2.6):
>>> sys.getsizeof('ciao')
28
>>> sys.getsizeof(u'ciao')
34
The Unicode object of course takes more memory space (very small difference for a very short string, obviously ;-).

There are two types of string in Python 2: the traditional str type and the newer unicode type. If you type a string literal without the u in front you get the old str type which stores 8-bit characters, and with the u in front you get the newer unicode type that can store any Unicode character.
The r doesn't change the type at all, it just changes how the string literal is interpreted. Without the r, backslashes are treated as escape characters. With the r, backslashes are treated as literal. Either way, the type is the same.
ur is of course a Unicode string where backslashes are literal backslashes, not part of escape codes.
You can try to convert a Unicode string to an old string using the str() function, but if there are any unicode characters that cannot be represented in the old string, you will get an exception. You could replace them with question marks first if you wish, but of course this would cause those characters to be unreadable. It is not recommended to use the str type if you want to correctly handle unicode characters.

'raw string' means it is stored as it appears. For example, '\' is just a backslash instead of an escaping.

Let me explain it simply:
In python 2, you can store string in 2 different types.
The first one is ASCII which is str type in python, it uses 1 byte of memory. (256 characters, will store mostly English alphabets and simple symbols)
The 2nd type is UNICODE which is unicode type in python. Unicode stores all types of languages.
By default, python will prefer str type but if you want to store string in unicode type you can put u in front of the text like u'text' or you can do this by calling unicode('text')
So u is just a short way to call a function to cast str to unicode. That's it!
Now the r part, you put it in front of the text to tell the computer that the text is raw text, backslash should not be an escaping character. r'\n' will not create a new line character. It's just plain text containing 2 characters.
If you want to convert str to unicode and also put raw text in there, use ur because ru will raise an error.
NOW, the important part:
You cannot store one backslash by using r, it's the only exception.
So this code will produce error: r'\'
To store a backslash (only one) you need to use '\\'
If you want to store more than 1 characters you can still use r like r'\\' will produce 2 backslashes as you expected.
I don't know the reason why r doesn't work with one backslash storage but the reason isn't described by anyone yet. I hope that it is a bug.

A "u" prefix denotes the value has type unicode rather than str.
Raw string literals, with an "r" prefix, escape any escape sequences within them, so len(r"\n") is 2. Because they escape escape sequences, you cannot end a string literal with a single backslash: that's not a valid escape sequence (e.g. r"\").
"Raw" is not part of the type, it's merely one way to represent the value. For example, "\\n" and r"\n" are identical values, just like 32, 0x20, and 0b100000 are identical.
You can have unicode raw string literals:
>>> u = ur"\n"
>>> print type(u), len(u)
<type 'unicode'> 2
The source file encoding just determines how to interpret the source file, it doesn't affect expressions or types otherwise. However, it's recommended to avoid code where an encoding other than ASCII would change the meaning:
Files using ASCII (or UTF-8, for Python 3.0) should not have a coding cookie. Latin-1 (or UTF-8) should only be used when a comment or docstring needs to mention an author name that requires Latin-1; otherwise, using \x, \u or \U escapes is the preferred way to include non-ASCII data in string literals.

Unicode string literals
Unicode string literals (string literals prefixed by u) are no longer used in Python 3. They are still valid but just for compatibility purposes with Python 2.
Raw string literals
If you want to create a string literal consisting of only easily typable characters like english letters or numbers, you can simply type them: 'hello world'. But if you want to include also some more exotic characters, you'll have to use some workaround.
One of the workarounds are Escape sequences. This way you can for example represent a new line in your string simply by adding two easily typable characters \n to your string literal. So when you print the 'hello\nworld' string, the words will be printed on separate lines. That's very handy!
On the other hand, sometimes you might want to include the actual characters \ and n into your string – you might not want them to be interpreted as a new line. Look at these examples:
'New updates are ready in c:\windows\updates\new'
'In this lesson we will learn what the \n escape sequence does.'
In such situations you can just prefix the string literal with the r character like this: r'hello\nworld' and no escape sequences will be interpreted by Python. The string will be printed exactly as you created it.
Raw string literals are not completely "raw"?
Many people expect the raw string literals to be raw in a sense that "anything placed between the quotes is ignored by Python". That is not true. Python still recognizes all the escape sequences, it just does not interpret them - it leaves them unchanged instead. It means that raw string literals still have to be valid string literals.
From the lexical definition of a string literal:
string ::= "'" stringitem* "'"
stringitem ::= stringchar | escapeseq
stringchar ::= <any source character except "\" or newline or the quote>
escapeseq ::= "\" <any source character>
It is clear that string literals (raw or not) containing a bare quote character: 'hello'world' or ending with a backslash: 'hello world\' are not valid.

Maybe this is obvious, maybe not, but you can make the string '\' by calling x=chr(92)
x=chr(92)
print type(x), len(x) # <type 'str'> 1
y='\\'
print type(y), len(y) # <type 'str'> 1
x==y # True
x is y # False

pandas to_dict() converts backslash to double backslash [duplicate]

When I create a string containing backslashes, they get duplicated:
>>> my_string = "why\does\it\happen?"
>>> my_string
'why\\does\\it\\happen?'
Why?

What you are seeing is the representation of my_string created by its __repr__() method. If you print it, you can see that you've actually got single backslashes, just as you intended:
>>> print(my_string)
why\does\it\happen?
The string below has three characters in it, not four:
>>> 'a\\b'
'a\\b'
>>> len('a\\b')
3
You can get the standard representation of a string (or any other object) with the repr() built-in function:
>>> print(repr(my_string))
'why\\does\\it\\happen?'
Python represents backslashes in strings as \\ because the backslash is an escape character - for instance, \n represents a newline, and \t represents a tab.
This can sometimes get you into trouble:
>>> print("this\text\is\not\what\it\seems")
this ext\is
ot\what\it\seems
Because of this, there needs to be a way to tell Python you really want the two characters \n rather than a newline, and you do that by escaping the backslash itself, with another one:
>>> print("this\\text\is\what\you\\need")
this\text\is\what\you\need
When Python returns the representation of a string, it plays safe, escaping all backslashes (even if they wouldn't otherwise be part of an escape sequence), and that's what you're seeing. However, the string itself contains only single backslashes.
More information about Python's string literals can be found at: String and Bytes literals in the Python documentation.

As Zero Piraeus's answer explains, using single backslashes like this (outside of raw string literals) is a bad idea.
But there's an additional problem: in the future, it will be an error to use an undefined escape sequence like \d, instead of meaning a literal backslash followed by a d. So, instead of just getting lucky that your string happened to use \d instead of \t so it did what you probably wanted, it will definitely not do what you want.
As of 3.6, it already raises a DeprecationWarning, although most people don't see those. It will become a SyntaxError in some future version.
In many other languages, including C, using a backslash that doesn't start an escape sequence means the backslash is ignored.
In a few languages, including Python, a backslash that doesn't start an escape sequence is a literal backslash.
In some languages, to avoid confusion about whether the language is C-like or Python-like, and to avoid the problem with \Foo working but \foo not working, a backslash that doesn't start an escape sequence is illegal.

r"string" b"string" u"string" Python 2 / 3 comparison

I already know r"string" in Python 2.7 often used for regex patterns. I also have seen u"string" for, I think, Unicode strings. Now with Python 3 we see b"string".
I have searched for these in different sources / questions, such as What does a b prefix before a python string mean?, but it's difficult to see the big picture of all these strings with prefixes in Python, especially with Python 2 vs 3.
Question: would you have a rule of thumb to remember the different types of strings with prefixes in Python? (or maybe a table with a column for Python 2 and one for Python 3?)
NB: I have read a few questions+answers but I haven't found an easy to remember comparison with all prefixes / Python 2+3

From the python docs for literals: https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
Bytes literals are always prefixed with 'b' or 'B'; they produce an
instance of the bytes type instead of the str type. They may only
contain ASCII characters; bytes with a numeric value of 128 or greater
must be expressed with escapes.
Both string and bytes literals may optionally be prefixed with a
letter 'r' or 'R'; such strings are called raw strings and treat
backslashes as literal characters. As a result, in string literals,
'\U' and '\u' escapes in raw strings are not treated specially. Given
that Python 2.x’s raw unicode literals behave differently than Python
3.x’s the 'ur' syntax is not supported.
and
A string literal with 'f' or 'F' in its prefix is a formatted string
literal; see Formatted string literals. The 'f' may be combined with
'r', but not with 'b' or 'u', therefore raw formatted strings are
possible, but formatted bytes literals are not.
So:
r means raw
b means bytes
u means unicode
f means format
The r and b were already available in Python 2, as such in many other languages (they are very handy sometimes).
Since the strings literals were not unicode in Python 2, the u-strings were created to offer support for internationalization. As of Python 3, u-strings are the default strings, so "..." is semantically the same as u"...".
Finally, from those, the f-string is the only one that isn't supported in Python 2.

u-strings if for unicode in python 2. Most probably you should forget this, if you're working with modern applications — default strings in python 3 is all unicode, and if you're migrating from python 2, you'll most probably use from __future__ import unicode_literals, which makes [almost] the same for python 2
b-strings is for raw bytes — have no idea of text, rather just stream of bytes. Rarely used as input for your source, most often as result of network or low-level code — reading data in binary format, unpacking archives, working with encryption libraries.
Moving from/to b-string to str done via
# python 3
>>> 'hēllö'.encode('utf-8')
b'h\xc4\x93ll\xc3\xb6'
>>> b'h\xc4\x93ll\xc3\xb6'.decode()
'hēllö'
# python 2 without __future__
>>> u'hēllö'.encode('utf-8')
'h\xc4\x93ll\xc3\xb6'
>>> 'h\xc4\x93ll\xc3\xb6'.decode('utf-8')
u'h\u0113ll\xf6' # this is correct representation
r-strings is not specifically for regex, this is "raw" string. Unlike regular string literals, r-string doesn't give any special meaning for escape characters. I.e. normal string 'abc\n' is 4 characters long, last char is "newline" special character. To provide it in literal, we're using escaping with \. For raw strings, r'abc\n' is 5-length string, last two characters is literally \ and n. Two places to see raw strings often:
regex patterns — to not mess escaping with actual special characters in patters
file path notations for windows systems, as windows family uses \ as delimeter, normal string literals will look like 'C:\\dir\\file', or '\\\\share\\dir', while raw would be nicer: r'C:\dir\file' and r'\\share\dir' respectively
One more notable is f-strings, which came to life with python 3.6 as simple and powerful way of formatting strings:
f'a equals {a} and b is {b}' will substitute variables a and b in runtime.

There are really only two types of string (or string-like object) in Python.
The first is 'Unicode' strings, which are a sequence of characters.
The second is bytes (or 'bytestrings'), which are a sequence of bytes.
The first is a series of letter characters found in the Unicode specification.
The second is a series of integers between 0 and 255 that are usually rendered to text using some assumed encoding such as ASCII or UTF-8 (which is a specification for encoding Unicode characters in a bytestream).
In Python 2, the default "my string" is a bytestring.
The prefix 'u' indicates a 'Unicode' string, e.g. u"my string".
In Python 3, 'Unicode' strings became the default, and thus "my string" is equivalent to u"my string".
To get the old Python 2 bytestrings, you use the prefix b"my string" (not in the oldest versions of Python 3).
There are two further prefixes, but they do not affect the type of string object, just the way it is interpreted.
The first is 'raw' strings which do not interpret escape characters such as \n or \t. For example, the raw string r"my_string\n" contains the literal backslash and 'n' character, while "my_string\n" contains a linebreak at the end of the line.
The second was introduced in the newest versions of Python 3: formatted strings with the prefix 'f'. In these, curly braces are used to show expressions to be interpreted. For example, the string in:
my_object = 'avocado'
f"my {0.5 + 1.0, my_object} string"
will be interpreted to "my (1.5, avocado) string" (where the comma created a tuple). This interpretation happens immediately when the code is read; there is nothing special subsequently about the string.
And finally, you can use the multiline string notation:
"""this is my
multiline
string"""
with 'r' or 'f' specifiers as you wish.
In Python 2, if you have used no prefix or only an 'r' prefix, it is a bytestring, and if you have used a 'u' prefix it is a Unicode string.
In Python 3, if you have used no prefix or only a combination of 'r', 'f' and 'u', it is a Unicode string. If you have used a 'b' prefix it is a bytestring. Using both 'b' and 'u' is obviously not allowed.

This is what I observed (seems confirmed by other answers):
Python 2 Python 3
-----------------------------------------------
"hello" b"hello"
b"hello" <=>
<type 'str'> <class 'bytes'>
-----------------------------------------------
u"hello" <=> "hello"
u"hello"
<type 'unicode'> <class 'str'>

os.path.normcase intrepreting backslashes as escaping sequence? [duplicate]

While asking this question, I realized I didn't know much about raw strings. For somebody claiming to be a Django trainer, this sucks.
I know what an encoding is, and I know what u'' alone does since I get what is Unicode.
But what does r'' do exactly? What kind of string does it result in?
And above all, what the heck does ur'' do?
Finally, is there any reliable way to go back from a Unicode string to a simple raw string?
Ah, and by the way, if your system and your text editor charset are set to UTF-8, does u'' actually do anything?

There's not really any "raw string"; there are raw string literals, which are exactly the string literals marked by an 'r' before the opening quote.
A "raw string literal" is a slightly different syntax for a string literal, in which a backslash, \, is taken as meaning "just a backslash" (except when it comes right before a quote that would otherwise terminate the literal) -- no "escape sequences" to represent newlines, tabs, backspaces, form-feeds, and so on. In normal string literals, each backslash must be doubled up to avoid being taken as the start of an escape sequence.
This syntax variant exists mostly because the syntax of regular expression patterns is heavy with backslashes (but never at the end, so the "except" clause above doesn't matter) and it looks a bit better when you avoid doubling up each of them -- that's all. It also gained some popularity to express native Windows file paths (with backslashes instead of regular slashes like on other platforms), but that's very rarely needed (since normal slashes mostly work fine on Windows too) and imperfect (due to the "except" clause above).
r'...' is a byte string (in Python 2.*), ur'...' is a Unicode string (again, in Python 2.*), and any of the other three kinds of quoting also produces exactly the same types of strings (so for example r'...', r'''...''', r"...", r"""...""" are all byte strings, and so on).
Not sure what you mean by "going back" - there is no intrinsically back and forward directions, because there's no raw string type, it's just an alternative syntax to express perfectly normal string objects, byte or unicode as they may be.
And yes, in Python 2.*, u'...' is of course always distinct from just '...' -- the former is a unicode string, the latter is a byte string. What encoding the literal might be expressed in is a completely orthogonal issue.
E.g., consider (Python 2.6):
>>> sys.getsizeof('ciao')
28
>>> sys.getsizeof(u'ciao')
34
The Unicode object of course takes more memory space (very small difference for a very short string, obviously ;-).

There are two types of string in Python 2: the traditional str type and the newer unicode type. If you type a string literal without the u in front you get the old str type which stores 8-bit characters, and with the u in front you get the newer unicode type that can store any Unicode character.
The r doesn't change the type at all, it just changes how the string literal is interpreted. Without the r, backslashes are treated as escape characters. With the r, backslashes are treated as literal. Either way, the type is the same.
ur is of course a Unicode string where backslashes are literal backslashes, not part of escape codes.
You can try to convert a Unicode string to an old string using the str() function, but if there are any unicode characters that cannot be represented in the old string, you will get an exception. You could replace them with question marks first if you wish, but of course this would cause those characters to be unreadable. It is not recommended to use the str type if you want to correctly handle unicode characters.

'raw string' means it is stored as it appears. For example, '\' is just a backslash instead of an escaping.

Let me explain it simply:
In python 2, you can store string in 2 different types.
The first one is ASCII which is str type in python, it uses 1 byte of memory. (256 characters, will store mostly English alphabets and simple symbols)
The 2nd type is UNICODE which is unicode type in python. Unicode stores all types of languages.
By default, python will prefer str type but if you want to store string in unicode type you can put u in front of the text like u'text' or you can do this by calling unicode('text')
So u is just a short way to call a function to cast str to unicode. That's it!
Now the r part, you put it in front of the text to tell the computer that the text is raw text, backslash should not be an escaping character. r'\n' will not create a new line character. It's just plain text containing 2 characters.
If you want to convert str to unicode and also put raw text in there, use ur because ru will raise an error.
NOW, the important part:
You cannot store one backslash by using r, it's the only exception.
So this code will produce error: r'\'
To store a backslash (only one) you need to use '\\'
If you want to store more than 1 characters you can still use r like r'\\' will produce 2 backslashes as you expected.
I don't know the reason why r doesn't work with one backslash storage but the reason isn't described by anyone yet. I hope that it is a bug.

A "u" prefix denotes the value has type unicode rather than str.
Raw string literals, with an "r" prefix, escape any escape sequences within them, so len(r"\n") is 2. Because they escape escape sequences, you cannot end a string literal with a single backslash: that's not a valid escape sequence (e.g. r"\").
"Raw" is not part of the type, it's merely one way to represent the value. For example, "\\n" and r"\n" are identical values, just like 32, 0x20, and 0b100000 are identical.
You can have unicode raw string literals:
>>> u = ur"\n"
>>> print type(u), len(u)
<type 'unicode'> 2
The source file encoding just determines how to interpret the source file, it doesn't affect expressions or types otherwise. However, it's recommended to avoid code where an encoding other than ASCII would change the meaning:
Files using ASCII (or UTF-8, for Python 3.0) should not have a coding cookie. Latin-1 (or UTF-8) should only be used when a comment or docstring needs to mention an author name that requires Latin-1; otherwise, using \x, \u or \U escapes is the preferred way to include non-ASCII data in string literals.

Unicode string literals
Unicode string literals (string literals prefixed by u) are no longer used in Python 3. They are still valid but just for compatibility purposes with Python 2.
Raw string literals
If you want to create a string literal consisting of only easily typable characters like english letters or numbers, you can simply type them: 'hello world'. But if you want to include also some more exotic characters, you'll have to use some workaround.
One of the workarounds are Escape sequences. This way you can for example represent a new line in your string simply by adding two easily typable characters \n to your string literal. So when you print the 'hello\nworld' string, the words will be printed on separate lines. That's very handy!
On the other hand, sometimes you might want to include the actual characters \ and n into your string – you might not want them to be interpreted as a new line. Look at these examples:
'New updates are ready in c:\windows\updates\new'
'In this lesson we will learn what the \n escape sequence does.'
In such situations you can just prefix the string literal with the r character like this: r'hello\nworld' and no escape sequences will be interpreted by Python. The string will be printed exactly as you created it.
Raw string literals are not completely "raw"?
Many people expect the raw string literals to be raw in a sense that "anything placed between the quotes is ignored by Python". That is not true. Python still recognizes all the escape sequences, it just does not interpret them - it leaves them unchanged instead. It means that raw string literals still have to be valid string literals.
From the lexical definition of a string literal:
string ::= "'" stringitem* "'"
stringitem ::= stringchar | escapeseq
stringchar ::= <any source character except "\" or newline or the quote>
escapeseq ::= "\" <any source character>
It is clear that string literals (raw or not) containing a bare quote character: 'hello'world' or ending with a backslash: 'hello world\' are not valid.

Maybe this is obvious, maybe not, but you can make the string '\' by calling x=chr(92)
x=chr(92)
print type(x), len(x) # <type 'str'> 1
y='\\'
print type(y), len(y) # <type 'str'> 1
x==y # True
x is y # False

Single letter before functions argument in Python [duplicate]

While asking this question, I realized I didn't know much about raw strings. For somebody claiming to be a Django trainer, this sucks.
I know what an encoding is, and I know what u'' alone does since I get what is Unicode.
But what does r'' do exactly? What kind of string does it result in?
And above all, what the heck does ur'' do?
Finally, is there any reliable way to go back from a Unicode string to a simple raw string?
Ah, and by the way, if your system and your text editor charset are set to UTF-8, does u'' actually do anything?

There's not really any "raw string"; there are raw string literals, which are exactly the string literals marked by an 'r' before the opening quote.
A "raw string literal" is a slightly different syntax for a string literal, in which a backslash, \, is taken as meaning "just a backslash" (except when it comes right before a quote that would otherwise terminate the literal) -- no "escape sequences" to represent newlines, tabs, backspaces, form-feeds, and so on. In normal string literals, each backslash must be doubled up to avoid being taken as the start of an escape sequence.
This syntax variant exists mostly because the syntax of regular expression patterns is heavy with backslashes (but never at the end, so the "except" clause above doesn't matter) and it looks a bit better when you avoid doubling up each of them -- that's all. It also gained some popularity to express native Windows file paths (with backslashes instead of regular slashes like on other platforms), but that's very rarely needed (since normal slashes mostly work fine on Windows too) and imperfect (due to the "except" clause above).
r'...' is a byte string (in Python 2.*), ur'...' is a Unicode string (again, in Python 2.*), and any of the other three kinds of quoting also produces exactly the same types of strings (so for example r'...', r'''...''', r"...", r"""...""" are all byte strings, and so on).
Not sure what you mean by "going back" - there is no intrinsically back and forward directions, because there's no raw string type, it's just an alternative syntax to express perfectly normal string objects, byte or unicode as they may be.
And yes, in Python 2.*, u'...' is of course always distinct from just '...' -- the former is a unicode string, the latter is a byte string. What encoding the literal might be expressed in is a completely orthogonal issue.
E.g., consider (Python 2.6):
>>> sys.getsizeof('ciao')
28
>>> sys.getsizeof(u'ciao')
34
The Unicode object of course takes more memory space (very small difference for a very short string, obviously ;-).

There are two types of string in Python 2: the traditional str type and the newer unicode type. If you type a string literal without the u in front you get the old str type which stores 8-bit characters, and with the u in front you get the newer unicode type that can store any Unicode character.
The r doesn't change the type at all, it just changes how the string literal is interpreted. Without the r, backslashes are treated as escape characters. With the r, backslashes are treated as literal. Either way, the type is the same.
ur is of course a Unicode string where backslashes are literal backslashes, not part of escape codes.
You can try to convert a Unicode string to an old string using the str() function, but if there are any unicode characters that cannot be represented in the old string, you will get an exception. You could replace them with question marks first if you wish, but of course this would cause those characters to be unreadable. It is not recommended to use the str type if you want to correctly handle unicode characters.

'raw string' means it is stored as it appears. For example, '\' is just a backslash instead of an escaping.

Let me explain it simply:
In python 2, you can store string in 2 different types.
The first one is ASCII which is str type in python, it uses 1 byte of memory. (256 characters, will store mostly English alphabets and simple symbols)
The 2nd type is UNICODE which is unicode type in python. Unicode stores all types of languages.
By default, python will prefer str type but if you want to store string in unicode type you can put u in front of the text like u'text' or you can do this by calling unicode('text')
So u is just a short way to call a function to cast str to unicode. That's it!
Now the r part, you put it in front of the text to tell the computer that the text is raw text, backslash should not be an escaping character. r'\n' will not create a new line character. It's just plain text containing 2 characters.
If you want to convert str to unicode and also put raw text in there, use ur because ru will raise an error.
NOW, the important part:
You cannot store one backslash by using r, it's the only exception.
So this code will produce error: r'\'
To store a backslash (only one) you need to use '\\'
If you want to store more than 1 characters you can still use r like r'\\' will produce 2 backslashes as you expected.
I don't know the reason why r doesn't work with one backslash storage but the reason isn't described by anyone yet. I hope that it is a bug.

A "u" prefix denotes the value has type unicode rather than str.
Raw string literals, with an "r" prefix, escape any escape sequences within them, so len(r"\n") is 2. Because they escape escape sequences, you cannot end a string literal with a single backslash: that's not a valid escape sequence (e.g. r"\").
"Raw" is not part of the type, it's merely one way to represent the value. For example, "\\n" and r"\n" are identical values, just like 32, 0x20, and 0b100000 are identical.
You can have unicode raw string literals:
>>> u = ur"\n"
>>> print type(u), len(u)
<type 'unicode'> 2
The source file encoding just determines how to interpret the source file, it doesn't affect expressions or types otherwise. However, it's recommended to avoid code where an encoding other than ASCII would change the meaning:
Files using ASCII (or UTF-8, for Python 3.0) should not have a coding cookie. Latin-1 (or UTF-8) should only be used when a comment or docstring needs to mention an author name that requires Latin-1; otherwise, using \x, \u or \U escapes is the preferred way to include non-ASCII data in string literals.

Unicode string literals
Unicode string literals (string literals prefixed by u) are no longer used in Python 3. They are still valid but just for compatibility purposes with Python 2.
Raw string literals
If you want to create a string literal consisting of only easily typable characters like english letters or numbers, you can simply type them: 'hello world'. But if you want to include also some more exotic characters, you'll have to use some workaround.
One of the workarounds are Escape sequences. This way you can for example represent a new line in your string simply by adding two easily typable characters \n to your string literal. So when you print the 'hello\nworld' string, the words will be printed on separate lines. That's very handy!
On the other hand, sometimes you might want to include the actual characters \ and n into your string – you might not want them to be interpreted as a new line. Look at these examples:
'New updates are ready in c:\windows\updates\new'
'In this lesson we will learn what the \n escape sequence does.'
In such situations you can just prefix the string literal with the r character like this: r'hello\nworld' and no escape sequences will be interpreted by Python. The string will be printed exactly as you created it.
Raw string literals are not completely "raw"?
Many people expect the raw string literals to be raw in a sense that "anything placed between the quotes is ignored by Python". That is not true. Python still recognizes all the escape sequences, it just does not interpret them - it leaves them unchanged instead. It means that raw string literals still have to be valid string literals.
From the lexical definition of a string literal:
string ::= "'" stringitem* "'"
stringitem ::= stringchar | escapeseq
stringchar ::= <any source character except "\" or newline or the quote>
escapeseq ::= "\" <any source character>
It is clear that string literals (raw or not) containing a bare quote character: 'hello'world' or ending with a backslash: 'hello world\' are not valid.

Maybe this is obvious, maybe not, but you can make the string '\' by calling x=chr(92)
x=chr(92)
print type(x), len(x) # <type 'str'> 1
y='\\'
print type(y), len(y) # <type 'str'> 1
x==y # True
x is y # False