Hướng dẫn python don t escape

I'm writing a Python script that accepts file paths as strings, parses them, appends a command name, and builds a list, which is then passed to subprocess.Popen[] for execution. This script is to handle both Unix and Windows file paths, and ultimately should run on both systems.

Nội dung chính

• Preventing Escape Sequence Interpretation
• Methods of Prevention
• How do I stop characters from escaping Python?
• How do I ignore an escape character in a string?
• What is escape characters in Python?
• How do you get rid of Backslashes in Python?

When I run this under Unix, if I give a Windows path that inadvertently contains an escape character [e.g. \Users\Administrator\bin], Python will interpret the embedded \b as the backspace character. I want to prevent that from happening.

As far as I know, there's no function or method to denote a string variable as a raw string. The 'r' modifier only works for string constants.

So far, the closest I've been able to get is this:

winpath = "C:\Users\Administrator\bin" 
winpath = winpath.replace['\b','\\b']
winpathlist = winpath.split['\\'] 

At this point, winpathlist should contain ['C:','Users','Administrator','bin'], not ['C','Users','Administrator\x08in'].

I can add additional calls to winpath.replace[] to handle the other escapes I might get -- \a, \f, \n, \r, \t, \v -- but not \x.

Is there a more pythonic way to do this?

Escape Sequence is a combination of characters [usually prefixed with an escape character], that has a non-literal character interpretation. Such that, the characters sequences which are considered as an escape sequence have a meaning other than the literal characters contained therein. Most Programming languages use a backslash \ as an escape character. This character is used as an escape sequence initiator, any character [one or more] following this is interpreted as an escape sequence. If an escape sequence is designated to a Non-Printable Character or a Control Code, then the sequence is called a control character. 

List of Escape Sequence in Python:

The above table is applicable for Python programming language, as different languages have different control sequences and control characters so the above table may not work in your programming language of choice. Ex. Windows Command Line interpreter uses a caret [ ^ ] to escape characters, and therefore the above table won’t be applicable there.

Escape Sequence Interpretation

Escape sequence interpretation is done, when a backslash is encountered within a string. After the encounter of a backslash [inside a string], any following character [with the [ \ ]] would be looked upon the aforementioned table. If a match is found then the sequence is omitted from the string, and its translation associated with the sequence is used. If a match is not found, then no lookup happens, and the control sequence is copied as it is. 

Example

Python3

print["I will go\tHome"]

print["See you\jtommorow"]

Output:

I will go    Home
See you\jtommorow

As seen in the above output, the first print statement produced an output where the \t got resolved into a vertical tab and is omitted in the output. On the other hand, in the second print statement, the \j persists, as no legal resolution for that sequence exists.

Preventing Escape Sequence Interpretation

There are instances where we don’t want the strings to behave in this way. In those cases, we generally want to preserve the backslashes. Some of the situations in which this may be required are:

• String contains a Network or Local path
• String contains regex, which would further be processed by the regex engine

Methods of Prevention

Method 1:

Consistently doubling the backslashes, also allows us to overcome such issues. In this method, we manually find every single backslash in the string and concatenate another backslash to it [at its immediate position]. Generally, a tedious method, and only advised if the string size is less. 

Python3

s = "I love to use \t instead of using 4 spaces"

print[s]

s = "I love to use \\t instead of using 4 spaces"

print[s]

Output:

I love to use      instead of using 4 spaces
I love to use \t instead of using 4 spaces

Method 2:

Using r’….’ or R’…..’ construct. Commonly referred to as raw strings, which is used to preserve the escape sequences as literals. Such that it does what the previous method did but automatically [does not require human intervention]. For turning a normal string into a raw string, prefix the string [before the quote] with an r or R. This is the method of choice for overcoming this escape sequence problem. 

Python3

s = "C:\Program Files\norton\appx"

print[s]

s = r"C:\Program Files\norton\appx"

print[s]

Output:

C:\Program Files
ortonppx
C:\Program Files\norton\appx

Problems due to escape characters may not always result in undesirable output, but also errors. For example, the below code upon execution will produce an error.

Python3

print["C:\Users\Desktop\JSON"]

Produces the following error

print[“C:\Users\Desktop\JSON”] 
       ^ 
SyntaxError: [unicode error] ‘unicodeescape’ codec can’t decode bytes in position 2-3: truncated \UXXXXXXXX escape
 

The error is caused because the  \U  in the string leads to the next 4 characters being treated as a 32-bit Hexadecimal value which would correspond to a Unicode code point. Which leads to an error as the next character is s  which are outside the base 16 range. 

How do I stop characters from escaping Python?

For turning a normal string into a raw string, prefix the string [before the quote] with an r or R. This is the method of choice for overcoming this escape sequence problem.

How do I ignore an escape character in a string?

An escape sequence is a set of characters used in string literals that have a special meaning, such as a new line, a new page, or a tab. For example, the escape sequence \n represents a new line character. To ignore an escape sequence in your search, prepend a backslash character to the escape sequence.

What is escape characters in Python?

An escape character is a backslash \ followed by the character you want to insert.

How do you get rid of Backslashes in Python?

Using replace[] Function to Remove Backslash from String in Python. The replace[] can be utilized to remove an escape sequence or just a backslash in Python by simply substituting it with space in Python.