Why would we want to use escape sequence characters like for example in this Python code:
print('It\'s alright.')
Why are we using this backslash to print a single quote when we can accomplish the same by using:
print("it's alright")
This is useful because you can do:
txt = 'in python you can have \'string\' or "string"'
print(txt)
No matter how many different kinds of quote you have, you may still need an escape mechanism now and then. Consider this:
If you want to use Python's "multiline string literal" you have to begin it and end it with a triple quote, which can be either """ or '''.
To put that into a string literal you are going to have to quote ' or ":
a = 'If you want to use Python\'s "multiline string literal" you have to begin it and end it with a triple quote, which can be either """ or \'\'\'.'.
a = "If you want to use Python's \"multiline string literal\" you have to begin it and end it with a triple quote, which can be either \"\"\" or '''."
a = """If you want to use Python's "multiline string literal" you have to begin it and end it with a triple quote, which can be either ""\" or '''."""
Having different quote types is a great programming convenience, making it easier and less error prone to put quotes and apostrophes in the data without having to jump through hoops. But it can't cover every case. If you need to convince yourself of this, experiment with those three lines at a command prompt and see if you can come up with a way to avoid backslashes. You will find you always need at least one.
Without further context, I can only take a guess and say that the person who wrote the first example, didn't know or wasn't aware of the fact that it's possible to use double-quotes "" for string literals in Python.
That's just a matter of style. Some people like to use single quotes to create string literals, and therefore they'll have to escape any single quotes it comes inside of their strings (same for double quotes). The following will raise a SyntaxError:
s = 'It's gonna be alright!'
s = "They used to call me "Big" but I was 4ft!"
So you may ask why they don't use " when their string have single quotes and ' when their string have double quotes? Yes, they can, but there are some unavoidable situations, such as Regex:
regexp = r"["']\w+["']"
Note that they can't use neither single nor double quotes to create the string, since both are present in the Regex. Therefore, they'll need to escape it.
In this case its not needed cuz you have used " " for the print statement.
case1) use: print(" It's alright.")
case2) use: print(' It\'s alright.')
Note the parenthesis used for the print statements.
You cant use ' directly in case2 cuz python would think that the string ends causing a SyntaxError.
In the code
txt = 'It\'s alright.'
you need the backslash(\) so python understands that the second apostrophe is a character of the string. Without the backslash, Python would interpret it as the character used to mark the end of the string.
When you use a ' at the start, python looks for a matching ' and considers whatever is present in between these quotes as a string.
But if you use a ' in the middle of the string, python considers that as the end of the string. And since there is no matching ' for the ' at the end of the string that results in a SyntaxError
The backslash () character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.
Refer the docs: https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
I am a trying to learn python and I have an issue where I encounter string that has both single and double quotes that I need to replace. My objective would be to split them to a list but still preserve the single and double quotes for other reasons. I tried the below but getting the errors
s=("I said, "hello" it's mine".replace(""",'`')).replace("'","^")
print(s)
File "<ipython-input-58-a5276888c42f>", line 1
s=("I said, "hello" it's mine".replace(""",'`')).replace("'","^")
^
SyntaxError: invalid syntax
Simply use a backslash to escape the double-quotes.
s = "I said, \"hello\" it's mine"
Or use a single-quoted string and escape the single-quote. (This is the canonical representation.)
s = 'I said, "hello" it\'s mine'
Or use triple-quotes - single or double.
s = """I said, "hello" it's mine"""
s = '''I said, "hello" it's mine'''
I'm using Python (and Pytumblr) and trying to extract a certain string from some returned data, but the string I am searching for includes ":" in it. Whenever I run my script I get the error:
File "myfile.py", line 22
if re.search('^ion': u'..', u'b', line) :
^
SyntaxError: invalid syntax
Here is my code:
import pytumblr
import re
returned = client.submission('blog') # get the submissions for a given blog
sch = open('returned')
for line in sch:
line = line.rstrip()
if re.search('^ion': u'..', u'b', line) :
print line
Is there another error in this code or is there a way to escape ":" that I don't know about? I'm pretty new to Python but I didn't think : needed to be escaped.
That's a syntax error because your colon is not part of the string. The single quote ' mark is closing off the string. Your first argument is being parsed as:
'^ion' - String 1: ^ion
: - Syntactical colon
u - The syntactical character u,
indicating you intend for the
following string literal to be
in unicode
'..' - String 2: ..
If you want your single quote at the end of ^ion to be a part of the string, you either need to escape that with a backslash '^ion\': or, alternatively, use double quotes around the string itself. Since Python accepts both single and double quotes for string literal markers, 'hello' and "hello" mean the same thing. Making '"hello world"' and "'hello world'" both legal strings.
If the regex is the pain point here, there's lots of literature and tooling out there to help. I recommend regex101
Try to use double quotes:
re.search("^ion': u'..', u'b", line):
Or escape ':
re.search('^ion\': u\'..\', u\'b', line):
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..).
I have
foo = '/DIR/abc'
and I want to convert it to
bar = '\\MYDIR\data\abc'
So, here's what I do in Python:
>>> foo = '/DIR/abc'
>>> bar = foo.replace(r'/DIR/',r'\\MYDIR\data\')
File "<stdin>", line 1
bar = foo.replace(r'/DIR/',r'\\MYDIR\data\')
^
SyntaxError: EOL while scanning string literal
If, however, I try to escape the last backslash by entering instead bar = foo.replace(r'/DIR/',r'\\MYDIR\data\\'), then I get this monstrosity:
>>> bar2
'\\\\MYDIR\\data\\\\abc'
Help! This is driving me insane.
The second argument should be a string, not a regex pattern:
foo.replace(r'/DIR/', '\\\\MYDIR\\data\\')
The reason you are encountering this is because of the behavior of the r"" syntax, Taking some explanation from the Python Documentation
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).
So you will need to use a normal escaped string for the last argument.
>>> foo = "/DIR/abc"
>>> print foo.replace(r"/DIR/", "\\\\MYDIR\\data\\")
\\MYDIR\data\abc
I simply put a r in front of / to change the forward slash.
inv_num = line.replace(r'/', '-')
Two problems:
A raw literal simply cannot end with a single backslash because it is interpreted as escaping the quote character. Therefore, use a regular (non-raw) literal with escapes: '\\\\MYDIR\\data\\'.
When displayed (using the repr style), strings will appear with escapes. Therefore, '\\\\' only has two actual backslashes. So, '\\\\MYDIR\\data\\\\abc' is really \\MYDIR\data\\abc.
path=path.replace(r"/","\") will replace path=C:/folder with path=C:\folder