I got a problem when appending a dict to list
data = []
path = "abc\cde"
data.append({"image": path})
print(data)
When I append the path to the image, the output of data is [{'image':'abc\def'}].
It contains two \ instead of one.
When typing text that contains slashes, use raw strings to avoid having some sequences be interpreted as special characters, e.g. "\n" in a python string is a single character that represents a new line.
>>> data = []
>>> data.append({"image": r'abc\cde'})
>>> data
[{'image': 'abc\\cde'}]
>>>
>>> data.append({"image": r'abc\nasdf'})
>>> data
[{'image': 'abc\\cde'}, {'image': 'abc\\nasdf'}]
When you see two slashes is because that's how python repr-esents a string with slashes safely, it's not the actual content.
>>> r'abc\cde'
'abc\\cde'
>>> r'abc\nasdf'
'abc\\nasdf'
In this way a text with special chars can be visualized in a compact way. If you want to see what the actual content of those strings looks like, print them:
>>> print(r'abc\cde')
abc\cde
>>> print(r'abc\nasdf')
abc\nasdf
>>> print('abc\cde')
abc\cde
>>> print('abc\nasdf')
abc
asdf
Using raw strings only applies to strings you type manually, it's a method to explain python how to interpret certain characters. If the string comes from e.g. a file or a stream, the "meaning" of its char is already defined.
Regarding your question on how to concatenate a raw string (again, a raw string is a normal string) with a variable, there's no difference.
>>> with_slash = r'abc\cde'
>>> wout_slash = 'asdf'
>>> with_slash + wout_slash
'abc\\cdeasdf'
>>> print(with_slash + wout_slash)
abc\cdeasdf
\ is an escape character. It allows you to use special symbols, for example a new line \n or tab \t. If you want a string to contain a literal \, make sure that you put another \ before it.
In your case, Python understands that you meant "abc\\cde" even though you did not escape \. If you had abc\nde, the result would be abc<line_break>de.
>>> a = "abc\\cde"
>>> a
'abc\\cde'
>>> list(a)
['a', 'b', 'c', '\\', 'c', 'd', 'e']
As you see, even though it looks like a double backslash, it is just one \ character.
More info: https://www.w3schools.com/python/gloss_python_escape_characters.asp
The additional backslash is Python escaping the single backslash. The actual value of your path string is unchanged, as you can see when the value of data[0]['image'] is printed.
data = []
path = 'abc\cde'
data.append({"image": path})
# output: abc\cde
print(data[0]['image'])
Related
I want to strip some unwanted symbols from my variable. In this case the symbols are backslashes. I am using a HEX number, and as an example I will show some short simple code down bellow. But I don't want python to convert my HEX to ASCII, how would I prevent this from happening.? I have some long shell codes for asm to work with later which are really long and removing \ by hand is a long process. I know there are different ways like using echo -e "x\x\x\x" > output etc, but my whole script will be written in python.
Thanks
>>> a = "\x31\xC0\x50\x68\x74\x76"
>>> b = a.strip("\\")
>>> print b
1�Phtv
>>> a = "\x31\x32\x33\x34\x35\x36"
>>> b = a.strip("\\")
>>> print b
123456
At the end I would like it to print my var:
>>> print b
x31x32x33x34x35x36
There are no backslashes in your variable:
>>> a = "\x31\xC0\x50\x68\x74\x76"
>>> print(a)
1ÀPhtv
Take newline for example: writing "\n" in Python will give you string with one character -- newline -- and no backslashes. See string literals docs for full syntax of these.
Now, if you really want to write string with such backslashes, you can do it with r modifier:
>>> a = r"\x31\xC0\x50\x68\x74\x76"
>>> print(a)
\x31\xC0\x50\x68\x74\x76
>>> print(a.replace('\\', ''))
x31xC0x50x68x74x76
But if you want to convert a regular string to hex-coded symbols, you can do it character by character, converting it to number ("\x31" == "1" --> 49), then to hex ("0x31"), and finally stripping the first character:
>>> a = "\x31\xC0\x50\x68\x74\x76"
>>> print(''.join([hex(ord(x))[1:] for x in a]))
'x31xc0x50x68x74x76'
There are two problems in your Code.
First the simple one:
strip() just removes one occurrence. So you should use replace("\\", ""). This will replace every backslash with "", which is the same as removing it.
The second problem is pythons behavior with backslashes:
To get your example working you need to append an 'r' in front of your string to indicate, that it is a raw string. a = r"\x31\xC0\x50\x68\x74\x76". In raw strings, a backlash doesn't escape a character but just stay a backslash.
>>> r"\x31\xC0\x50\x68\x74\x76"
'\\x31\\xC0\\x50\\x68\\x74\\x76'
So I am making a program that takes a text file, breaks it into words, then writes the list to a new text file.
The issue I am having is I need the strings in the list to be with double quotes not single quotes.
For example
I get this ['dog','cat','fish'] when I want this ["dog","cat","fish"]
Here is my code
with open('input.txt') as f:
file = f.readlines()
nonewline = []
for x in file:
nonewline.append(x[:-1])
words = []
for x in nonewline:
words = words + x.split()
textfile = open('output.txt','w')
textfile.write(str(words))
I am new to python and haven't found anything about this.
Anyone know how to solve this?
[Edit: I forgot to mention that i was using the output in an arduino project that required the list to have double quotes.]
You cannot change how str works for list.
How about using JSON format which use " for strings.
>>> animals = ['dog','cat','fish']
>>> print(str(animals))
['dog', 'cat', 'fish']
>>> import json
>>> print(json.dumps(animals))
["dog", "cat", "fish"]
import json
...
textfile.write(json.dumps(words))
Most likely you'll want to just replace the single quotes with double quotes in your output by replacing them:
str(words).replace("'", '"')
You could also extend Python's str type and wrap your strings with the new type changing the __repr__() method to use double quotes instead of single. It's better to be simpler and more explicit with the code above, though.
class str2(str):
def __repr__(self):
# Allow str.__repr__() to do the hard work, then
# remove the outer two characters, single quotes,
# and replace them with double quotes.
return ''.join(('"', super().__repr__()[1:-1], '"'))
>>> "apple"
'apple'
>>> class str2(str):
... def __repr__(self):
... return ''.join(('"', super().__repr__()[1:-1], '"'))
...
>>> str2("apple")
"apple"
>>> str2('apple')
"apple"
In Python, double quote and single quote are the same. There's no different between them. And there's no point to replace a single quote with a double quote and vice versa:
2.4.1. String and Bytes literals
...In plain English: Both types of literals can be enclosed in matching single quotes (') or double quotes ("). They can also be enclosed in matching groups of three single or double quotes (these are generally referred to as triple-quoted strings). The backslash () character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character...
"The issue I am having is I need the strings in the list to be with double quotes not single quotes." - Then you need to make your program accept single quotes, not trying to replace single quotes with double quotes.
I have a file of strings one per line in which non-ascii characters have been escaped with decimal code points. One example line is:
mj\\195\\164ger
(The double backslashes are in the file exactly as printed)
I would like to process this string to produce
mjäger
. Conventionally, python uses hexadecimal escapes rather than decimal escapes (e.g., the above string would be written as mj\xc3\xa4ger, which python can decode:
>>> by=b'mj\xc3\xa4ger'
>>> by.decode('utf-8')
'mjäger'
Python, however, doesn't recognize the decimal escape right away.
I have written a method that correctly manipulates the strings to produce hexadecimal escapes, but these escapes are themselves escaped. How can I get python to process these hexadecimal escapes to create the final string?
import re
hexconst=["0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"]
escapes=re.compile(r"\\[0-9]{3}")
def dec2hex(matchobj):
dec=matchobj.group(0)
dec=int(dec[1:])
digit1=dec//16 #integer division
digit2=dec%16
hex="\\x" + hexconst[digit1] + hexconst[digit2]
return hex
line=r'mj\195\164ger'
print(escapes.sub(dec2hex,line)) #Outputs mj\xc3\xa4ger
What is the final step I'm missing to convert the output of the above from mj\xc3\xa4ger to mjäger? Thanks!
It's much easier. re.sub() can take a callback function instead of a replacement string as an argument:
>>> import re
>>> line=r'mj\195\164ger'
>>> def replace(match):
... return chr(int(match.group(1)))
>>> regex = re.compile(r"\\(\d{1,3})")
>>> new = regex.sub(replace, line)
>>> new
'mj\xc3\xa4ger'
>>> print new
mjäger
In Python 3, strings are Unicode strings, so if you're working with encoded input (like UTF-8 encoded content), then you need to use the proper type which is bytes:
>>> line = rb'mj\195\164ger'
>>> regex = re.compile(rb"\\(\d{1,3})")
>>> def replace(match):
... return int(match.group(1)).to_bytes(1, byteorder="big")
>>> new = regex.sub(replace, line)
>>> new
b'mj\xc3\xa4ger'
>>> print(new.decode("utf-8"))
mjäger
I have a set of strings that are read from a file say ['\x1\p1', '\x2\p2', '\x3\p3', ... etc.].
When I read them into variables and print them the strings displayed as ['\\x1\\p1', '\\x2\\p2', '\\x3\\p3', ... etc.]. I understand that the variable is represented as '\x1\p1', ... etc. internally, but when it is displayed it is displayed with double slash.
but now I want to search and replace the elements of this list in the sentence, i.e say if \x1\p1 is in the sentence "How are you doing \x1\p1" then replace '\x1\p1' with 'Y'. But the replace method does not work in this case! wonder why?
Let me explain further:
my text file (codes.txt) has entries \xs1\x32, \xs2\x54 delimited by new line. so when I read it using
with open('codes') as codes:
code_list = codes.readlines()
next, I do lets say code_list_element_1 = code_list[1].rstrip()
when I print code_list_element_1, it displays as '\\xs1\\x32'
Next, let me target string be target_string = 'Hi! my name is \xs1\x32'
now I want to replace code_list_element_1 which is supposed to be \xs1\x32 in the target_string with say 'Y'
So, I tried code_list_element_1 in target_string. I get False
Next, instead of reading the codes from a text file I initialized a variable find_me = '\xs1\x32'
now, I try find_me in target_string. I get True
and hence target_string.replace(find_me,"Y") displays what I want: "Hi! my name is Y"
You are looking at a string representation that can be pasted back into Python; the backslashes are doubled to make sure the values are not interpreted as escape sequences (such as \n, meaning a newline, or \xfe, meaning the byte with value 254, hex FE).
If you are building new string values, you also need to use those doubled backslashes to prevent Python from seeing escape sequences where there are none, or use raw string literals:
>>> '\\x1\\p1'
'\\x1\\p1'
>>> r'\x1\p1'
'\\x1\\p1'
For this specific example, not handling the backslashes properly actually results in an exception:
>>> '\x1\p1'
ValueError: invalid \x escape
because Python expects to find two hex digits after a \x escape.
raw strings (those prefixed by r are very useful for backslash-itis.
In [9]: a=r"How are you doing \x1\p1"
In [10]: a
Out[10]: 'How are you doing \\x1\\p1'
In [11]: a.replace(r'\x1\p1', 'Y')
Out[11]: 'How are you doing Y'
In [12]:
i've the following code:
import re
key = re.escape('#one #two #some #tests #are #done')
print(key)
key = key.split()
print(key)
and the following output:
\#one\ \#two\ \#some\ \#tests\ \#are\ \#done
['\\#one\\', '\\#two\\', '\\#some\\', '\\#tests\\', '\\#are\\', '\\#done']
How come the backslashes are duplicated? I just want them once in my list, because i would like to use this list in a regular expression.
Thanks in advance! John
There is only one backslash each, but when printing the repr of the strings, they are duplicated (escaped) - just as you would need to duplicate them when using a string to build a regex. So everything is fine.
For example:
>>> len("\\")
1
>>> len("\\n")
2
>>> len("\n")
1
>>> print "\\n"
\n
>>> print "\n"
>>>
The \ character is an escape character, that is a character that changes the meaning of the subsequent character[s]. For example the "n" character is simply an "n". But if you escape it like "\n" it becomes the "newline" character. So, if you need to use a \ literal, you need to escape it with... itself: \\
The backslashes are not duplicated. To realize this, try to do:
for element in key:
print element
And you will see this output:
\#one\
\#two\
\#some\
\#tests\
\#are\
\#done
When you have printed whole list, the python used representation where strings are printed not as they are, but they are printed as python expression (notice the quotes "", they are not in the strings)
To actually encode string containing backslash, you need to duplicate that backslash. That is it.
When you convert a list to a string (e.g. to print it), it calls repr on each object contained in the list. That's why you get the quotes and extra backslashes in your second line of output. Try this:
s = "\\a string with an escaped backslash"
print s # prints: \a string with an escaped backslash
print repr(s) # prints: '\\a string with an escaped backslash'
The repr call puts quotes around the string, and shows the backslash escapes.