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# Python - Strings

Strings are amongst the most popular types in Python. We can create them simply by enclosing characters in quotes. Python treats single quotes the same as double quotes.

Creating strings is as simple as assigning a value to a variable. For example:

    var1 = 'Hello World!'
    var2 = "Python Programming"

# Accessing Values in Strings:

Python does not support a character type; these are treated as strings of length one, thus also considered a substring.

To access substrings, use the square brackets for slicing along with the index or indices to obtain your substring:

## Example:

    #!/usr/bin/python

    var1 = 'Hello World!'
    var2 = "Python Programming"

    print "var1[0]: ", var1[0]
    print "var2[1:5]: ", var2[1:5]

This will produce following result:

    var1[0]:  H
    var2[1:5]:  ytho

# Updating Strings:

You can "update" an existing string by (re)assigning a variable to another string. The new value can be related to its previous value or to a completely different string altogether.

## Example:

    #!/usr/bin/python

    var1 = 'Hello World!'

    print "Updated String :- ", var1[:6] + 'Python'

This will produce following result:

    Updated String :-  Hello Python

# Escape Characters:

Following table is a list of escape or non-printable characters that can be represented with backslash notation.

NOTE: In a doublequoted string, an escape character is interpreted; in a singlequoted string, an escape character is preserved.

Backslash notation | Hexadecimal character | Description
--- | --- | ---
\a | 0x07 | Bell or alert
\b | 0x08 | Backspace
\cx |  | Control-x
\C-x |  | Control-x
\e | 0x1b | Escape
\f | 0x0c | Formfeed
\M-\C-x |  | Meta-Control-x
\n | 0x0a | Newline
\nnn |  | Octal notation, where n is in the range 0.7
\r | 0x0d | Carriage return
\s | 0x20 | Space
\t | 0x09 | Tab
\v | 0x0b | Vertical tab
\x |  | Character x
\xnn |  | Hexadecimal notation, where n is in the range 0.9, a.f, or A.F

# String Special Operators:

Assume string variable a holds 'Hello' and variable b holds 'Python' then:

Operator | Description | Example
--- | --- | ---
+ | Concatenation - Adds values on either side of the operator | a + b will give HelloPython
* | Repetition - Creates new strings, concatenating multiple copies of the same string | a*2 will give -HelloHello
[] | Slice - Gives the character from the given index | a[1] will give e
[ : ] | Range Slice - Gives the characters from the given range | a[1:4] will give ell
in | Membership - Returns true if a character exists in the given string | H in a will give 1
not in | Membership - Returns true if a character does not exist in the given string | M not in a will give 1
r/R | Raw String - Suppress actual meaning of Escape characters. The syntax for raw strings is exactly the same as for normal strings with the exception of the raw string operator, the letter "r," which precedes the quotation marks. The "r" can be lowercase (r) or uppercase (R) and must be placed immediately preceding the first quote mark. | print r'\n' prints \n and print R'\n' prints \n
% | Format - Performs String formatting | See at next section

# String Formatting Operator:

One of Python's coolest features is the string format operator %. This operator is unique to strings and makes up for the pack of having functions from C's printf() family.

## Example:

    #!/usr/bin/python

    print "My name is %s and weight is %d kg!" % ('Zara', 21)

This will produce following result:

    My name is Zara and weight is 21 kg!

Here is the list of complete set of symbols which can be used along with %:

Format Symbol | Conversion
--- | ---
%c | character
%s | string conversion via str() prior to formatting
%i | signed decimal integer
%d | signed decimal integer
%u | unsigned decimal integer
%o | octal integer
%x | hexadecimal integer (lowercase letters)
%X | hexadecimal integer (UPPERcase letters)
%e | exponential notation (with lowercase 'e')
%E | exponential notation (with UPPERcase 'E')
%f | floating point real number
%g | the shorter of %f and %e
%G | the shorter of %f and %E

Other supported symbols and functionality are listed in the following table:

Symbol | Functionality
--- | ---
* | argument specifies width or precision
- | left justification
+ | display the sign
<sp> | leave a blank space before a positive number
# | add the octal leading zero ( '0' ) or hexadecimal leading '0x' or '0X', depending on whether 'x' or 'X' were used.
0 | pad from left with zeros (instead of spaces)
% | '%%' leaves you with a single literal '%'
(var) | mapping variable (dictionary arguments)
m.n. | m is the minimum total width and n is the number of digits to display after the decimal point (if appl.)

# Triple Quotes:

Python's triple quotes comes to the rescue by allowing strings to span multiple lines, including verbatim NEWLINEs, TABs, and any other special characters.

The syntax for triple quotes consists of three consecutive single or double quotes.

    #!/usr/bin/python

    para_str = """this is a long string that is made up of
    several lines and non-printable characters such as
    TAB ( \t ) and they will show up that way when displayed.
    NEWLINEs within the string, whether explicitly given like
    this within the brackets [ \n ], or just a NEWLINE within
    the variable assignment will also show up.
    """
    print para_str;

This will produce following result. Note how every single special character has been converted to its printed form, right down to the last NEWLINE at the end of the string between the "up." and closing triple quotes. Also note that NEWLINEs occur either with an explicit carriage return at the end of a line or its escape code (\n):

    this is a long string that is made up of
    several lines and non-printable characters such as
    TAB (    ) and they will show up that way when displayed.
    NEWLINEs within the string, whether explicitly given like
    this within the brackets [
     ], or just a NEWLINE within
    the variable assignment will also show up.

# Raw String:

Raw strings don't treat the backslash as a special character at all. Every character you put into a raw string stays the way you wrote it:

    #!/usr/bin/python

    print 'C:\\nowhere'

This would print following result:

    C:\nowhere

Now let's make use of raw string. We would put expression in r'expression' as follows:

    #!/usr/bin/python

    print r'C:\\nowhere'

This would print following result:

    C:\\nowhere

# Unicode String:

Normal strings in Python are stored internally as 8-bit ASCII, while Unicode strings are stored as 16-bit Unicode. This allows for a more varied set of characters, including special characters from most languages in the world. I'll restrict my treatment of Unicode strings to the following:

    #!/usr/bin/python

    print u'Hello, world!'

This would print following result:

    Hello, world!

As you can see, Unicode strings use the prefix u, just as raw strings use the prefix r.

# Built-in String Methods:

Python includes following string method:

SN | Methods with Description
--- | ---
1 | capitalize() Capitalizes first letter of string
2 | center(width, fillchar) Returns a space-padded string with the original string centered to a total of width columns
3 | count(str, beg= 0,end=len(string)) Counts how many times str occurs in string, or in a substring of string if starting index beg and ending index end are given
3 | decode(encoding='UTF-8',errors='strict') Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding.
4 | encode(encoding='UTF-8',errors='strict') Returns encoded string version of string; on error, default is to raise a ValueError unless errors is given with 'ignore' or 'replace'.
5 | endswith(suffix, beg=0, end=len(string)) Determines if string or a substring of string (if starting index beg and ending index end are given) ends with suffix; Returns true if so, and false otherwise
6 | expandtabs(tabsize=8) Expands tabs in string to multiple spaces; defaults to 8 spaces per tab if tabsize not provided
7 | find(str, beg=0 end=len(string)) Determine if str occurs in string, or in a substring of string if starting index beg and ending index end are given; returns index if found and -1 otherwise
8 | index(str, beg=0, end=len(string)) Same as find(), but raises an exception if str not found
9 | isa1num() Returns true if string has at least 1 character and all characters are alphanumeric and false otherwise
10 | isalpha() Returns true if string has at least 1 character and all characters are alphabetic and false otherwise
11 | isdigit() Returns true if string contains only digits and false otherwise
12 | islower() Returns true if string has at least 1 cased character and all cased characters are in lowercase and false otherwise
13 | isnumeric() Returns true if a unicode string contains only numeric characters and false otherwise
14 | isspace() Returns true if string contains only whitespace characters and false otherwise
15 | istitle() Returns true if string is properly "titlecased" and false otherwise
16 | isupper() Returns true if string has at least one cased character and all cased characters are in uppercase and false otherwise
17 | join(seq) Merges (concatenates) the string representations of elements in sequence seq into a string, with separator string
18 | len(string) Returns the length of the string
19 | ljust(width[, fillchar]) Returns a space-padded string with the original string left-justified to a total of width columns
20 | lower() Converts all uppercase letters in string to lowercase
21 | lstrip() Removes all leading whitespace in string
22 | maketrans() Returns a translation table to be used in translate function.
23 | max(str) Returns the max alphabetical character from the string str
24 | min(str) Returns the min alphabetical character from the string str
25 | replace(old, new [, max]) Replaces all occurrences of old in string with new, or at most max occurrences if max given
26 | rfind(str, beg=0,end=len(string)) Same as find(), but search backwards in string
27 | rindex( str, beg=0, end=len(string)) Same as index(), but search backwards in string
28 | rjust(width,[, fillchar]) Returns a space-padded string with the original string right-justified to a total of width columns.
29 | rstrip() Removes all trailing whitespace of string
30 | split(str="", num=string.count(str)) Splits string according to delimiter str (space if not provided) and returns list of substrings; split into at most num substrings if given
31 | splitlines( num=string.count('\n')) Splits string at all (or num) NEWLINEs and returns a list of each line with NEWLINEs removed
32 | startswith(str, beg=0,end=len(string)) Determines if string or a substring of string (if starting index beg and ending index end are given) starts with substring str; Returns true if so, and false otherwise
33 | strip([chars]) Performs both lstrip() and rstrip() on string
34 | swapcase() Inverts case for all letters in string
35 | title() Returns "titlecased" version of string, that is, all words begin with uppercase, and the rest are lowercase
36 | translate(table, deletechars="") Translates string according to translation table str(256 chars), removing those in the del string
37 | upper() Converts lowercase letters in string to uppercase
38 | zfill (width) Returns original string leftpadded with zeros to a total of width characters; intended for numbers, zfill() retains any sign given (less one zero)
39 | isdecimal() Returns true if a unicode string contains only decimal characters and false otherwise

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