How to ignore escape character in python

This one is a bit tricky I think.

if I have:

a = "fwd"
b = "\fwd"

how can I ignore the "\" so something like

print(a in b)

can evaluate to True?

asked Apr 14, 2016 at 13:00

5

You don't have fwd in b. You have wd, preceded by ASCII codepoint 0C, the FORM FEED character. That's the value Python puts there when you use a \f escape sequence in a regular string literal.

Double the backslash if you want to include a backslash or use a raw string literal:

b = '\\fwd'
b = r'\fwd'

Now a in b works:

>>> 'fwd' in '\\fwd'
True
>>> 'fwd' in r'\fwd'
True

See the String literals documentation:

Unless an 'r' or 'R' prefix is present, escape sequences in strings are interpreted according to rules similar to those used by Standard C. The recognized escape sequences are:

[...]

\f ASCII Formfeed (FF)

jfs

382k179 gold badges944 silver badges1614 bronze badges

answered Apr 14, 2016 at 13:05

Martijn Pieters♦Martijn Pieters

986k274 gold badges3877 silver badges3236 bronze badges

One way of doing it using raw strings:

>>> a = "fwd"
>>> b = "\fwd"
>>> a in b
False
>>> a = r"fwd"
>>> b = r"\fwd"
>>> a in b
True

The relevant docs

answered Apr 14, 2016 at 13:04

jDojDo

3,8021 gold badge10 silver badges30 bronze badges

You need to "escape" the backslash, as in:

b = '\\fwd'

Otherwise, it reads the single backslash + f as an ASCII character (a formfeed).

Here's an example.

>>> a='fwd'
>>> b='\fwd'
>>> c='\\fwd'
>>> a in b
False
>>> a in c
True

answered Apr 14, 2016 at 13:05

rajah9rajah9

10.9k4 gold badges42 silver badges55 bronze badges

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?

How do you ignore in Python?

What does \r do in Python?

How do you remove special characters in Python?