Develop Reference

Understanding file locations in python - unexpected errors

I am learning python 3.3 in windows 7. I have a two text files - lines.txt and raven.txt in a folder. Both contain the same text for the first example.
When I try to access ravens, using the code below, I get the error -
OSError: [Errno 22] Invalid argument: 'C:\\Python\raven.txt'
I know that the above error can be fixed by using an escape character like this -
C:\\Python\\raven.txt
C:\Python\\raven.txt
Why do both methods work ? Strangely, when I access lines.txt in the same folder, I get no error ! Why ?
import re
def main():
print('')
fh = open('C:\Python\lines.txt')
for line in fh:
if re.search('(Len|Neverm)ore', line):
print(line, end = '')
if __name__ == '__main__':main()
Also, when I use the line below, I get a completely different error - TypeError: embedded NUL character. Why ?
fh = open('C:\Python\Exercise Files\09 Regexes\raven.txt')
I can rectify this by using \ before every \ in the file path.

\r is an escape character, but \l is not. So, lines is interpreted as lines while raven is interpreted as aven, since \r is escaped.
In [1]: len('\l')
Out[1]: 2
In [2]: len('\r')
Out[2]: 1
You should always escape backslashes with \\. In cases your string doesn't have quotes, you can also use raw strings:
In [9]: len(r'\r')
Out[9]: 2
In [10]: r'\r'
Out[10]: '\\r'
See: https://docs.python.org/3/reference/lexical_analysis.html

maybe you can use raw string.
just like this open(r'C:\Python\Exercise Files\09 Regexes\raven.txt').
When an r' orR' prefix is present, backslashes are still used to
quote the following character, but all backslashes are left in the
string. For example, the string literal r"\n" consists of two
characters: a backslash and a lowercase `n'. 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 value 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.

You can actually use forward slashes instead of backward ones, that way you don't have to escape them at all, which would save you a lot of headaches. Like this: 'C:/Python/raven.txt', I can guarantee that it works on Windows.

Slash replacement inside a raw string

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.

python replace single backslash with double backslash [duplicate]

This question already has answers here:
How can I put an actual backslash in a string literal (not use it for an escape sequence)?
(4 answers)
Closed 7 months ago.
In python, I am trying to replace a single backslash ("\") with a double backslash("\"). I have the following code:
directory = string.replace("C:\Users\Josh\Desktop\20130216", "\", "\\")
However, this gives an error message saying it doesn't like the double backslash. Can anyone help?

No need to use str.replace or string.replace here, just convert that string to a raw string:
>>> strs = r"C:\Users\Josh\Desktop\20130216"
^
|
notice the 'r'
Below is the repr version of the above string, that's why you're seeing \\ here.
But, in fact the actual string contains just '\' not \\.
>>> strs
'C:\\Users\\Josh\\Desktop\\20130216'
>>> s = r"f\o"
>>> s #repr representation
'f\\o'
>>> len(s) #length is 3, as there's only one `'\'`
3
But when you're going to print this string you'll not get '\\' in the output.
>>> print strs
C:\Users\Josh\Desktop\20130216
If you want the string to show '\\' during print then use str.replace:
>>> new_strs = strs.replace('\\','\\\\')
>>> print new_strs
C:\\Users\\Josh\\Desktop\\20130216
repr version will now show \\\\:
>>> new_strs
'C:\\\\Users\\\\Josh\\\\Desktop\\\\20130216'

Let me make it simple and clear. Lets use the re module in python to escape the special characters.
Python script :
import re
s = "C:\Users\Josh\Desktop"
print s
print re.escape(s)
Output :
C:\Users\Josh\Desktop
C:\\Users\\Josh\\Desktop
Explanation :
Now observe that re.escape function on escaping the special chars in the given string we able to add an other backslash before each backslash, and finally the output results in a double backslash, the desired output.
Hope this helps you.

Use escape characters: "full\\path\\here", "\\" and "\\\\"

In python \ (backslash) is used as an escape character. What this means that in places where you wish to insert a special character (such as newline), you would use the backslash and another character (\n for newline)
With your example string you would notice that when you put "C:\Users\Josh\Desktop\20130216" in the repl you will get "C:\\Users\\Josh\\Desktop\x8130216". This is because \2 has a special meaning in a python string. If you wish to specify \ then you need to put two \\ in your string.
"C:\\Users\\Josh\\Desktop\\28130216"
The other option is to notify python that your entire string must NOT use \ as an escape character by pre-pending the string with r
r"C:\Users\Josh\Desktop\20130216"
This is a "raw" string, and very useful in situations where you need to use lots of backslashes such as with regular expression strings.
In case you still wish to replace that single \ with \\ you would then use:
directory = string.replace(r"C:\Users\Josh\Desktop\20130216", "\\", "\\\\")
Notice that I am not using r' in the last two strings above. This is because, when you use the r' form of strings you cannot end that string with a single \
Why can't Python's raw string literals end with a single backslash?
https://pythonconquerstheuniverse.wordpress.com/2008/06/04/gotcha-%E2%80%94-backslashes-are-escape-characters/

Maybe a syntax error in your case,
you may change the line to:
directory = str(r"C:\Users\Josh\Desktop\20130216").replace('\\','\\\\')
which give you the right following output:
C:\\Users\\Josh\\Desktop\\20130216

The backslash indicates a special escape character. Therefore, directory = path_to_directory.replace("\", "\\") would cause Python to think that the first argument to replace didn't end until the starting quotation of the second argument since it understood the ending quotation as an escape character.
directory=path_to_directory.replace("\\","\\\\")

Given the source string, manipulation with os.path might make more sense, but here's a string solution;
>>> s=r"C:\Users\Josh\Desktop\\20130216"
>>> '\\\\'.join(filter(bool, s.split('\\')))
'C:\\\\Users\\\\Josh\\\\Desktop\\\\20130216'
Note that split treats the \\ in the source string as a delimited empty string. Using filter gets rid of those empty strings so join won't double the already doubled backslashes. Unfortunately, if you have 3 or more, they get reduced to doubled backslashes, but I don't think that hurts you in a windows path expression.

You could use
os.path.abspath(path_with_backlash)
it returns the path with \

Use:
string.replace(r"C:\Users\Josh\Desktop\20130216", "\\", "\\")
Escape the \ character.

how to replace back slash character with empty string in python [duplicate]

This question already has answers here:
How can I print a single backslash?
(4 answers)
Closed 7 months ago.
I am trying replace a backslash '\' in a string with the following code
string = "<P style='TEXT-INDENT'>\B7 </P>"
result = string.replace("\",'')
result:
------------------------------------------------------------
File "<ipython console>", line 1
result = string.replace("\",'')
^
SyntaxError: EOL while scanning string literal
Here i don't need the back slashes because actually i am parsing an xml file which has a tag in the above format, so if backslashes are there it is displaying invalid token during parsing
Can i know how to replace the backslashes with empty string in python

We need to specify that we want to replace a string that contains a single backslash. We cannot write that as "\", because the backslash is escaping the intended closing double-quote. We also cannot use a raw string literal for this: r"\" does not work.
Instead, we simply escape the backslash using another backslash:
result = string.replace("\\","")

The error is because you did not add a escape character to your '\', you should give \\ for backslash (\)
In [147]: foo = "a\c\d" # example string with backslashes
In [148]: foo
Out[148]: 'a\\c\\d'
In [149]: foo.replace('\\', " ")
Out[149]: 'a c d'
In [150]: foo.replace('\\', "")
Out[150]: 'acd'

In Python, as explained in the documentation:
The backslash () character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.
So, in order to replace \ in a string, you need to escape the backslash itself with another backslash, thus:
>>> "this is a \ I want to replace".replace("\\", "?")
'this is a ? I want to replace'

Using regular expressions:
import re
new_string = re.sub("\\\\", "", old_string)
The trick here is that "\\\\" is a string literal describing a string containing two backslashes (each one is escaped), then the regex engine compiles that into a pattern that will match one backslash (doing a separate layer of unescaping).

Adding a solution if string='abcd\nop.png'
result = string.replace("\\","")
This above won't work as it'll give result='abcd\nop.png'.
Here if you see \n is a newline character. So we have to replace backslah char in raw string(as there '\n' won't be detected)
string.encode('unicode_escape')
result = string.replace("\\", "")
#result=abcdnop.png

You need to escape '\' with one extra backslash to compare actually with \.. So you should use '\'..
See Python Documentation - section 2.4 for all the escape sequences in Python.. And how you should handle them..

It's August 2020.
Python 3.8.1
Pandas 1.1.0
At this point in time I used both the double \ backslash AND the r.
df.replace([r'\\'], [''], regex=True, inplace=True)
Cheers.

re module - what is the r notation?

I am try to understand this code:
Var1 = re.compile(r"nothing is (\d+)").search
i am want to see what is the affect of the r notation right after the ( sign on the \d.
i know that \d mean to find decimal numbers (the \ mean that d has a special meaning), is that mean that without the r notation i would use \ ? if so why ?
i know that at the bash shell it is enough to put just one \ .
i have read here: http://docs.python.org/library/re.html#raw-string-notation
but i'm can't understand how it affects the snippets code above.
Thanks.

It's a raw string literal. It changes backslashes to be treated literally (almost, see below). This is particularly useful when writing regular expressions as they often contain backslashes and if you use ordinary string literal you may have to escape the backslashes, making the regular expression harder to read.
Without the r your code would look like this:
f = re.compile("nothing is (\\d+)").search
Note that not escaping the backslashes also works in this case because '\d' is not a valid escape sequence:
f = re.compile("nothing is (\d+)").search
However, relying on this behaviour may result in errors unless you (and everyone who has to maintain your code) can memorize the list of allowable escape sequences.
The rules for raw string literals are:
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. For example, the string literal r"\n" consists of two characters: a backslash and a lowercase 'n'. 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.

Has nothing to do with re.
Python string literals may start with r; these are called "raw strings" and backslash escapes are (mostly) not interpreted.