There is geometry in the humming of the strings, there is music in the spacing of the spheres.

Pythagoras
How to Change Those Guitar Strings. | Superprof
We are not talking Guitar Strings

First, i  trust everyone is safe.

Second, this is the SB[3].  We are going to be covering some basics in Python of what constitutes a string, modifying a string, and explaining several string manipulation methods.

I also realized in the last Snake_Byte that i didn’t reference the book that i randomly open and choose the subject for the Snake_Byte.  I will be adding that as a reference at the end of the blog.

Strings can be used to represent just about anything.

They can be binary values of bytes, internet addresses, names, and Unicode for international localization.

They are part of a larger class of objects called sequences.  In fact, python strings are immutable sequences.  Immutability means you cannot change the sequence or the sequence does not change over time.

The most simplistic string is an empty string:

a = “ “ # with either singe or double quotes

There are numerous expression operations, modules, and methods for string manipulations.  

Python also supports much more advanced operations for those familiar with regular expressions (regex) it supports them via reEven more advanced operations are available such as XML parsing and the like.

Python is really into strings.

So let us get literal, shall we?

For String Literals there are countless ways to create and manipulate strings in your code:

Single Quotes:

a = `i w”ish you water’

Double Quotes;

A = “i w’ish you water”

Even triple quotes (made me think of the “tres commas” episode from Silicon Valley)

A = ```... i wish you water ```

Single and double quotes are by far the most used.  I prefer double quotes probably due to the other languages i learned before Python.

Python also supports the liberal use of backslashes aka escape sequences.  I’m sure everyone is familiar with said character `\`.  

Escape sequences let us embed bytecodes into strings that are otherwise difficult to type.

So let’s see here:

s = 't\nc\nt\njr'
print (s)
t
c
t
jr

So here i used ‘\n’ to represent the byte containing the binary value for newline character which is ASCII code 10.  There are several accessible representations:

‘\a\’ # bell

‘\b\’ #backspace

‘\f’ # formfeed for all the dot matrix printers we use 

‘\r’ #carriage return

You can even do different Unicode hex values:

‘\Uhhhhhhhh’ #32 bit hex count the number of h’s

With respect to binary file representations of note in Python 3.0 binary file content is represented by an actual byte string with operations similar to normal strings. 

One big difference between Python and another language like C is that that the zero (null) byte doesn’t terminate and in fact, there are no character string terminations in Python.  Also, the strings length and text reside in memory. 

s = 'a\0b\0c'
print (s)
len (s)
abc
5

So what can we do with strings in Python?

Well, we can concatenate:

a = "i wish"
print(len (a))
b = " you water"
print (len(b))
c = a + b
print (len(c))
print (c)
6
10
16
i wish you water

So adding two strings creates a new string object and a new address in memory.  It is also a form of operator overloading in place.  The ‘ + ‘ sign does the job for strings and can add numerics.  You also don’t have to “pre-declare” and allocate memory which is one of the advantages of Python.  In Python, computational processes are described directly by the programmer. A declarative language abstracts away procedural details however Python isn’t purely declarative which is outside the scope of the blog.  

So what else?  Well, there is indexing and slicing:

Strings are ordered collections of characters ergo we can access the characters by the positions within the ordering.

You access the component by providing a numerical offset via square brackets this is indexing.  

S = "i wish you water"
print (S[0], S[4], S[-1])
i s r

Since we can index we can slice:

S = "i wish you water"
print (S[1:3], S[2:10], S[9:10])
w wish you u

Slicing is a particular form of indexing more akin to parsing where you analyze the structure.

Python once again creates a new object containing the contiguous section identified by the offset pair.  It is important to note the left offset is taken to be the inclusive lower bound and the right is the non-inclusive upper bound. The inclusive definition is important here:  Including the endpoints of an interval. For example, “the interval from 1 to 2, inclusive” means the closed interval written [1, 2].  This means Python fetches all items from the lower bound up to but not including the upper bound. 

What about changing a string?

Let’s try it:

S = "i wish you water"
S[0] = "x"
---------------------------------------------------------------------------
TypeError Traceback (most recent call last) <ipython-input-67-a6fd56571822> 
in <module> 1 S = "i wish you water" ----> 2 S[0] = "x"
TypeError: 'str' object does not support item assignment

Ok, what just happened?  Well, remember the word immutable? You cannot change it in place.

To change a string you need to create a new one through various methods.  In the current case we will use a combination of concatenation, indexing, and slicing to bring it all together:

S = "i wish you water"
S = 'x ' + S[2]  +  S[3:17]
print (S)
x wish you water

This brings us to methods.

Stings in Python provide a set of methods that implements much more complex text processing.  Just like in other languages a method or function takes parameters and returns a value. A “method” is a specific type of function: it must be part of a “class”, so has access to the class’ member variables. A function is usually discrete and all variables must be passed into the function.

Given the previous example there is a replace method:

S = "i wish you water"
S = S.replace ('i wish you water', 'x wish you water')
print (S)
x wish you water

Let’s try some other methods;

# captialize the first letter in a string:
S = "i wish you water"
S.capitalize()
'I wish you water'

# capitalize all the letters in a string:
S = "i wish you water"
S.upper()
'I WISH YOU WATER'

# check if the string is a digit:
S = "i wish you water"
S.isdigit()
False

# check it again:
S = "999"
S.isdigit()
TRUE

# strip trailing spaces in a string:
S = "i wish you water     "
x = S.rstrip()
print("of all fruits", x, "is my favorite") 
of all fruits i wish you water is my favorite

The list is seemingly endless.  

One more caveat emptor you should use stings methods, not the original string module that was deprecated in Python 3.0

We could in fact write multiple chapters on strings by themselves.  However, this is supposed to be a little nibble of what the Snake language can offer.  We have added the reference that we used to make this blog at the end.  I believe it is one of the best books out there for learning Python.

Until Then,

#iwishyouwater

Tctjr

References:

Learning Python by Mark Lutz

Muzak To Blog By:  Mr. Robot Vol1 Original Television Soundtrack

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