When I write print('\') or print("\") or print("'\'"), Python doesn't print the backslash \ symbol. Instead it errors for the first two and prints '' for the third. What should I do to print a backslash?
This question is about producing a string that has a single backslash in it. This is particularly tricky because it cannot be done with raw strings. For the related question about why such a string is represented with two backslashes, see Why do backslashes appear twice?. For including literal backslashes in other strings, see using backslash in python (not to escape).
You need to escape your backslash by preceding it with, yes, another backslash:
print("\\")
And for versions prior to Python 3:
print "\\"
The \ character is called an escape character, which interprets the character following it differently. For example, n by itself is simply a letter, but when you precede it with a backslash, it becomes \n, which is the newline character.
As you can probably guess, \ also needs to be escaped so it doesn't function like an escape character. You have to... escape the escape, essentially.
See the Python 3 documentation for string literals.
A hacky way of printing a backslash that doesn't involve escaping is to pass its character code to chr:
>>> print(chr(92))
\
print(fr"\{''}")
or how about this
print(r"\ "[0])
For completeness: A backslash can also be escaped as a hex sequence: "\x5c"; or a short Unicode sequence: "\u005c"; or a long Unicode sequence: "\U0000005c". All of these will produce a string with a single backslash, which Python will happily report back to you in its canonical representation - '\\'.
I found the sequence \newline in a list of escape sequences in the python documentation. I wonder how it is used and for what. At least in my interpreter it seems this is just interpreted as '\n' + 'ewline':
>>> print('\newline')
ewline
It refers to the actual newline character - the one with character code "10" (0x0a) - not the text sequence "newline".
So, an example is like:
print("a\
b")
Here, the backslash is succeeded by the newline, inside a string, and what is printed is just "ab" with nothing apart.
it differs from \n - in here, the characer following the backslash is n (0x6e), and this sequence is translated to \x0a on parsing the string. On \<newline>, the source string contains the \x0a character and that is replaced by an empty string.
Maybe the documentation on that page would be more clear if it would read \<newline> instead of just \newline.
The documentation you are alluding to is explaining how a backslash followed by a literal newline is ignored, as if the next line were physically joined with the line on which the starting backslash was found.
The string \newline' has no special meaning; it is exactly what you say you think it is.
I'm writing a quick Python script to do a bit of inspection on some of our Hibernate mapping files. I'm trying to use this bit of Python to get the table name of a POJO, whether or not its class path is fully defined:
searchObj = re.search(r'<class name="(.*\\.|)' + pojo + '".*table="(.*?)"', contents)
However - say pojo is 'MyObject' - the regex is not matching it to this line:
<class name="com.place.package.MyObject" table="my_cool_object" dynamic-insert="true" dynamic-update="true">
If I print the string (while stopped in Pdb) I'm searching with, I see this:
'<class name="(.*\\\\.|)MyObject".*table="(.*?)"'
I'm quite confused as to what's going wrong here. For one, I was under the impression that the 'r' prefix made it so that the backslashes wouldn't be escaped. Even so, if I remove one of the backslashes such that my search string is this:
searchObj = re.search(r'<class name="(.*\.|)' + pojo + '".*table="(.*?)"', contents)
And the string searched becomes
'<class name="(.*\\.|)MyObject".*table="(.*?)"'
It still doesn't return a match. What's going wrong here? The regex expression I'm intending to use works on regex101.com (with just one backslash in the apparently problematic area.) Any idea what is going wrong here?
Given this:
re.search(r'<class name="(.*\\.|)' + pojo + '".*table="(.*?)"', contents)
The the first part of the pattern is interpreted like this:
1. class name=" a literal string beginning with c and ending with "
2. ( the beginning of a group
3. .* zero or more of any characters
4. \\ a literal single slash
5. . any single character
6. OR
7. nothing
8. ) end of the group
Since the string you're searching for does not have a literal backslash, it won't match.
If what you intend is for \\. to mean "a literal period", you need a single backslash since it is inside a raw string: \.
Also, ending the group with a pipe seems weird. I'm not sure what you think that's accomplishing. If you mean to say "any number of characters ending in a dot, or nothing", you can do that with (.*\.)?, since the ? means "zero or one of the preceding match".
This seems to work for me:
import re
contents1 = '''<class name="com.place.package.MyObject" table="my_cool_object" dynamic-insert="true" dynamic-update="true">'''
contents2 = '''<class name="MyObject" table="my_cool_object" dynamic-insert="true" dynamic-update="true">'''
pojo="MyObject"
pattern = r'<class name="(.*\.)?' + pojo + '.*table="(.*?)"'
assert(re.search(pattern, contents1))
assert(re.search(pattern, contents2))
On Pythex, I tried this regex:
<class name="(.*)\.MyObject" table="([^"]*)"
on this string:
<class name="com.place.package.MyObject" table="my_cool_object" dynamic-insert="true" dynamic-update="true">
and got these two match captures:
com.place.package
my_cool_object
So I think in your case, this line
searchObj = re.search(r'<class name="(.*)\.' + pojo + '"table="([^"]*)"', contents)
will produce the result you want.
About the confusing backslashes – you add two and then four show up, on the Python documentation 7.2. re — Regular expression operations it explains that r'' is “raw string notation”, used to circumvent Python’s regular character escaping, which uses a backslash. So:
'\\' means “a string composed of one backslash”, since the first backslash in the string escapes the second backslash. Python sees the first backslash and thinks, ‘the next character is a special one’; then it sees the second and says, ‘the special character is an actual backslash’. It’s stored as a single character \. If you ask Python to print this, it will escape the output and show you "\\".
r'\\' means “a string composed of two actual backslashes. It’s stored as character \ followed by character \. If you ask Python to print this, it will escape the output and show you "\\\\".
Technically, any odd number of backslashes, as described in the documentation.
>>> r'\'
File "<stdin>", line 1
r'\'
^
SyntaxError: EOL while scanning string literal
>>> r'\\'
'\\\\'
>>> r'\\\'
File "<stdin>", line 1
r'\\\'
^
SyntaxError: EOL while scanning string literal
It seems like the parser could just treat backslashes in raw strings as regular characters (isn't that what raw strings are all about?), but I'm probably missing something obvious.
The whole misconception about python's raw strings is that most of people think that backslash (within a raw string) is just a regular character as all others. It is NOT. The key to understand is this python's tutorial sequence:
When an 'r' or 'R' prefix is present, a character following a
backslash is included in the string without change, and all
backslashes are left in the string
So any character following a backslash is part of raw string. Once parser enters a raw string (non Unicode one) and encounters a backslash it knows there are 2 characters (a backslash and a char following it).
This way:
r'abc\d' comprises a, b, c, \, d
r'abc\'d' comprises a, b, c, \, ', d
r'abc\'' comprises a, b, c, \, '
and:
r'abc\' comprises a, b, c, \, ' but there is no terminating quote now.
Last case shows that according to documentation now a parser cannot find closing quote as the last quote you see above is part of the string i.e. backslash cannot be last here as it will 'devour' string closing char.
The reason is explained in the part of that section which I highlighted in bold:
String quotes can be escaped with a
backslash, but the backslash remains
in the string; for example, r"\"" is a
valid string literal consisting of two
characters: a backslash and a double
quote; r"\" is not a valid string
literal (even a raw string cannot end
in an odd number of backslashes).
Specifically, a raw string cannot end
in a single backslash (since the
backslash would escape the following
quote character). Note also that a
single backslash followed by a newline
is interpreted as those two characters
as part of the string, not as a line
continuation.
So raw strings are not 100% raw, there is still some rudimentary backslash-processing.
That's the way it is! I see it as one of those small defects in python!
I don't think there's a good reason for it, but it's definitely not parsing; it's really easy to parse raw strings with \ as a last character.
The catch is, if you allow \ to be the last character in a raw string then you won't be able to put " inside a raw string. It seems python went with allowing " instead of allowing \ as the last character.
However, this shouldn't cause any trouble.
If you're worried about not being able to easily write windows folder pathes such as c:\mypath\ then worry not, for, you can represent them as r"C:\mypath", and, if you need to append a subdirectory name, don't do it with string concatenation, for it's not the right way to do it anyway! use os.path.join
>>> import os
>>> os.path.join(r"C:\mypath", "subfolder")
'C:\\mypath\\subfolder'
In order for you to end a raw string with a slash I suggest you can use this trick:
>>> print r"c:\test"'\\'
test\
It uses the implicit concatenation of string literals in Python and concatenates one string delimited with double quotes with another that is delimited by single quotes. Ugly, but works.
Another trick is to use chr(92) as it evaluates to "\".
I recently had to clean a string of backslashes and the following did the trick:
CleanString = DirtyString.replace(chr(92),'')
I realize that this does not take care of the "why" but the thread attracts many people looking for a solution to an immediate problem.
Since \" is allowed inside the raw string. Then it can't be used to identify the end of the string literal.
Why not stop parsing the string literal when you encounter the first "?
If that was the case, then \" wouldn't be allowed inside the string literal. But it is.
The reason for why r'\' is syntactical incorrect is that although the string expression is raw the used quotes (single or double) always have to be escape since they would mark the end of the quote otherwise. So if you want to express a single quote inside single quoted string, there is no other way than using \'. Same applies for double quotes.
But you could use:
'\\'
Another user who has since deleted their answer (not sure if they'd like to be credited) suggested that the Python language designers may be able to simplify the parser design by using the same parsing rules and expanding escaped characters to raw form as an afterthought (if the literal was marked as raw).
I thought it was an interesting idea and am including it as community wiki for posterity.
Naive raw strings
The naive idea of a raw string is
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
and it will mean itself.
Unfortunately, this does not work, because if the whatever
happens to contain a quote, the raw string would end at that point.
It is simply impossible that I can put "whatever I want"
between fixed delimiters, because some of it could look like
the terminating delimiter -- no matter what that delimiter is.
Real-world raw strings (variant 1)
One possible approach to this problem would be to say
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
This restriction sounds harsh, until one recognizes that
Python's large offering of quotes can accommodate most situations
with this rule. The following are all valid Python quotes:
'
"
'''
"""
With this many possibilities for the delimiter, almost anything
can be made to work.
About the only exception would be if the string
literal is supposed to contain a complete list of all allowed
Python quotes.
Real-world raw strings (variant 2, as in Python)
Python, however, takes a different route using
an extended version of the above rule.
It effectively states
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
If I insist on including a quote, even that is allowed,
but I have to put a backslash before it.
So the Python approach is, in a sense, even more liberal
than variant 1 above -- but it has the side effect of
"mis"interpreting the closing quote as part of the string
if the last intended character of the string is a backslash.
Variant 2 is not helpful:
If I want the quote in my string,
but not the backslash, the allowed version of my string literal
will not be what I need.
However, given the three different other kinds of quotes I have
at my disposal, I will probably just pick one of those and my
problem will be solved -- so this is not problematic case.
The problematic case is this one:
If I want my string to end with a backslash, I am at a loss.
I need to resort to concatenating a non-raw string literal
containing the backslash.
Conclusion
After writing this, I go with several of the other posters
that variant 1 would have been easier to understand and to accept
and therefore more pythonic. That's life!
Comming from C it pretty clear to me that a single \ works as escape character allowing you to put special characters such as newlines, tabs and quotes into strings.
That does indeed disallow \ as last character since it will escape the " and make the parser choke. But as pointed out earlier \ is legal.
some tips :
1) if you need to manipulate backslash for path then standard python module os.path is your friend. for example :
os.path.normpath('c:/folder1/')
2) if you want to build strings with backslash in it BUT without backslash at the END of your string then raw string is your friend (use 'r' prefix before your literal string). for example :
r'\one \two \three'
3) if you need to prefix a string in a variable X with a backslash then you can do this :
X='dummy'
bs=r'\ ' # don't forget the space after backslash or you will get EOL error
X2=bs[0]+X # X2 now contains \dummy
4) if you need to create a string with a backslash at the end then combine tip 2 and 3 :
voice_name='upper'
lilypond_display=r'\DisplayLilyMusic \ ' # don't forget the space at the end
lilypond_statement=lilypond_display[:-1]+voice_name
now lilypond_statement contains "\DisplayLilyMusic \upper"
long live python ! :)
n3on
Despite its role, even a raw string cannot end in a single
backslash, because the backslash escapes the following quote
character—you still must escape the surrounding quote character to
embed it in the string. That is, r"...\" is not a valid string
literal—a raw string cannot end in an odd number of backslashes.
If you need to end a raw string with a single backslash, you can use
two and slice off the second.
Given the confusion around the arbitrary-seeming restriction against an odd number of backslashes at the end of a Python raw-string, it's fair to say that this is a design mistake or legacy issue originating in a desire to have a simpler parser.
While workarounds (such as r'C:\some\path' '\\' yielding 'C:\\some\\path\\' (in Python notation) or C:\some\path\ (verbatim)) are simple, it's counterintuitive to be needing them. For comparison, let's have a look at C++ and Perl.
In C++, we can straightforwardly use raw string literal syntax
#include <iostream>
int main() {
std::cout << R"(Hello World!)" << std::endl;
std::cout << R"(Hello World!\)" << std::endl;
std::cout << R"(Hello World!\\)" << std::endl;
std::cout << R"(Hello World!\\\)" << std::endl;
}
to get the following output:
Hello World!
Hello World!\
Hello World!\\
Hello World!\\\
If we want to use the closing delimiter (above: )) within the string literal, we can even extend the syntax in an ad-hoc way to R"delimiterString(quotedMaterial)delimiterString". For example, R"asdf(some random delimiters: ( } [ ] { ) < > just for fun)asdf" produces the string some random delimiters: ( } [ ] { ) < > just for fun in the output. (Ain't that a good use of "asdf"!)
In Perl, this code
my $str = q{This is a test.\\};
print ($str);
print ("This is another test.\n");
will output the following: This is a test.\This is another test.
Replacing the first line by
my $str = q{This is a test.\};
would lead to an error message: Can't find string terminator "}" anywhere before EOF at main.pl line 1.
However, Perl treating a pre-delimiter \ as an escape character doesn't prevent the user from having an odd number of backslashes at the end of the resulting string; eg to place 3 backslashes \\\ into the end of $str, simply end the code with 6 backslashes: my $str = q{This is a test.\\\\\\};. Importantly, while we need to double the backslashes in the input, there is no Python-like inconsistent-seeming syntactic restriction.
Another way of looking at things is that these 3 languages use different ways to address the parsing issue of interaction between escape characters and closing delimiters:
Python: disallows an odd number of backslashes just before the closing delimiter; a simple workaround is r'stringWithoutFinalBackslash' '\\'
C++: allows essentially¹ everything between the delimiters
Perl: allows essentially² everything between the delimiters, but backslashes need to be consistently doubled
¹ The custom delimiterString itself cannot be more than 16 characters long, but that's hardly a limitation.
² If you need the delimiter itself, just escape it with \.
However, to be fair in a comparison to Python, we need to acknowledge that (1) C++ didn't have such string literals until C++11 and is famously hard to parse and (2) Perl is even harder to parse.
I encountered this problem and found a partial solution which is good for some cases. Despite python not being able to end a string with a single backslash, it can be serialized and saved in a text file with a single backslash at the end. Therefore if what you need is saving a text with a single backslash on you computer, it is possible:
x = 'a string\\'
x
'a string\\'
# Now save it in a text file and it will appear with a single backslash:
with open("my_file.txt", 'w') as h:
h.write(x)
BTW it is not working with json if you dump it using python's json library.
Finally, I work with Spyder, and I noticed that if I open the variable in spider's text editor by double clicking on its name in the variable explorer, it is presented with a single backslash and can be copied to the clipboard that way (it's not very helpful for most needs but maybe for some..).
Just a simple question concerning raw string, regex pattern and replacement:
I have a string variable defined as follow:
> print repr(foo)
'\n\t\t\n\t\tIf (GUTIAttach>=1) //In case of GUTI attach Enodeb should not ask RRCUecapa again\n\t\tUECapInfo;//Mps("( \\"rat_Type\\":0 \\"ueCapabilitiesRAT_Container\\":hex:011c0000000080 )");
My problem are characters "(" and ")", I want to replace them by "\(" and "\)" inside the raw string because it will be used after as a regular expression pattern.
I tried to use this method:
foo_tmp= [inc.replace(')', '\)') for inc in foo]
foo_tmp= [inc.replace('(', '\)') for inc in foo_tmp]
foo = "".join(foo_tmp)
the result gives:
> print repr(foo)
'\n\t\t\n\t\tIf \\(GUTIAttach>=1\\) //In case of GUTI attach Enodeb should not ask RRCUecapa again\n\t\t{\n\t\t\tUECapInfo;//Mps\\("\\( \\"rat_Type\\":0 \\"ueCapabilitiesRAT_Container\\":hex:011c0000000080 \\)"\\);
Characters "(" and ")" have been replaced by "\\(" and "//)" instead of "\(" and "\)".
That's a bit unexpected for me, so do you know how I can proceed to get just a single slash without changing the other part of the string?
Note: The method .decode('string_escape') is also not working due to the rest of string. Double slashes already present in the original raw string must not change.
Thanks a lot for your help
Use the re.escape() function to escape regular expression meta characters for you.
What you are seeing is otherwise perfectly normal Python behaviour; you are looking at a python literal representation; the output can be pasted back into a Python interpreter and recreate the value. As such, anything that could be interpreted as an escape code is escaped for you; a single \ would normally be doubled to prevent it being interpreted as the start of an escape sequence:
>>> '\('
'\\('
>>> print '\\('
\(
You can see this at work in other places in your foo string; the \n character combination represents a newline character, not two separate characters \ and n. If you wanted to include a literal \ and n in the text, you'd have to double the backslash to \\n. Further on into the value of foo you'll find \\", which is a single backslash followed by a " quote.