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. Following is a simple example:
#!/usr/bin/python
var1 = 'Hello World!'
var2 = "Python Programming"
print "var1[0]: ", var1[0]
print "var2[1:5]: ", var2[1:5]
When the above code is executed, it produces the 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. Following is a simple example:
#!/usr/bin/python
var1 = 'Hello World!'
print "Updated String :- ", var1[:6] + 'Python'
When the above code is executed, it produces the 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.
An escape character gets interpreted; in asinglequoted as well as doublequoted strings.
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 giveHelloPython
*
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 giveell
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 - Suppresses 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 andprint 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. Following is a simple example:
#!/usr/bin/python
print "My name is %s and weight is %d kg!" % ('Zara', 21)
When the above code is executed, it produces the 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 (UPPERcaseletters)
%e
exponential notation (with lowercase 'e')
%E
exponential notation (withUPPERcase '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;
When the above code is executed, it produces the 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 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'
When the above code is executed, it produces the 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'
When the above code is executed, it produces the 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!'
When the above code is executed, it produces the 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:
ليست هناك تعليقات:
إرسال تعليق