DEV Community: libertycodervice

String formatting in Python

libertycodervice — Wed, 04 Mar 2020 18:45:21 +0000

You can do two ways of string formatting in Python. The most easy way is to use f-strings or formatted strings. The other way is similar to the printf function in the C programming language.

The printf way:

>>> print('this is %d %s pet' % (1,'hungry'))
this is 1 hungry pet
>>>

The f-strings way (has an f in front of the string):

>>> n = 1
>>> s = 'hungry'
>>> print(f'this is {n} {s} pet')
this is 1 hungry pet
>>>

Obviously the f-strings way is better, but you may find code with the old C style way. So in the C printf way, what is this %?

It lets you output a type of variable. For a string %s, for a number %d.

>>> 
>>> name = 'goofy'
>>> 'my name is %s ' % name
'my name is goofy '
>>> 'that will be %d dollars please' % 5
'that will be 5 dollars please'
>>>

So you know %d and %s` string formatting codes, but there are others.

Code	meaning
%s	string (or any object)
%r	s, but with repr, rather than the str
%c	character
%d	a decimal integer
%i	integer
%u	Unsigned integer
%o	octal integer
%x	hexadecimal integer
%X	x, but the print uppercase
%e	floating-point index
%E	e, but prints uppercase
%f	floating decimal
%F	floating decimal
%g	e or f floating point
%G	floating point e or f

>>> a = 678
>>> print('integer %i is a number' % a)
integer 678 is a number
>>>

If you want a float, change it:

>>> a = 1.23456
>>> print('float has value of %f ' %a)
float has value of 1.234560 
>>>

You can use multiple variables in a string:

>>> name = 'goofy'
>>> age = 50
>>> print('I am %s and I am %d years old' % (name,age))
I am goofy and I am 50 years old
>>>

Things you must know about Python strings

libertycodervice — Wed, 04 Mar 2020 18:29:54 +0000

Python strings let you work with text. If you define text e.g. Python string, you can do so in two ways:

>>> s = 'abc'
>>> s
'abc'
>>>
>>> s = "abc"
>>> s
'abc'
>>>

So single quoted string is the same as double quoted. You can be use this interchangeably, but preferably just pick one.

You can even introduce quotes inside the strings:

      >>> 'abc "d'," abc'd "
      ( 'abc "d'," abc'd ")

Python can automatically merge strings. To do so you should use a + instead of a comma. A comma will give you a tuple.

So not this:

>>> s = "hello ","world"
>>> s
('hello ', 'world')

Also, in the above example: the comma in the middle of the string, so the final form is a tuple rather than a string

But this concatenates strings:

>>> s = "hello " + "world"
>>> s
'hello world'
>>>

Use the escape character for special characters

Python supports many special characters. An escape character starts with a slash.

>>> print('hello\nhello\n',end='')
hello
hello
>>>

Common escape characters:

Escape	sense
\	Continuous, then the top line
\	Backslash
\'	Single quotes
\"	Double quotes
\n	Newline
\a	Bell
\b	back
\f	feed
\r	returned
\t	horizontal tab
\v	vertical tab
\n{id}	unicode database id
\Uhhhh	unicode16 bit hexadecimal value
\Uhhhh	unicode32 bit hexadecimal value
\Xhh	hexadecimal values
\Ooo	octal value
\0	Null

remove trailing spaces

libertycodervice — Wed, 04 Mar 2020 17:05:21 +0000

If you have a Python text string, and want trailing spaces removed you can do that with the method .strip(), .lstrip() and .rstrip().

That removes the spaces before and after the string.

Example

You can use strip, lstrip, rstrip methods. Like the name suggests, lstrip strips left of the string, rstrip strips right of the string and strip just strips the string of spaces.

First create a string surrounded by spaces:

>>> a = "abc".center(30)
>>> a
'             abc              '
>>>

To do a left strip, call the method lstrip(). This removes spaces to the left of the string:

>>> b = a.lstrip()
>>> b
'abc              '

The results of a right strip, call rstrip() which removes spaces to the right of the string:

>>> 
>>> c = a.rstrip()
>>> c
'             abc'

To do a general strip, that removes all spaces on both sides:

>>> 
>>> d = a.strip()
>>> d
'abc'
>>> 
>>>

The strip() function only does trailing spaces, not spaces inside the string itself.

>>> a = "    aaaa  bbbb ccc    dddd   eee           "
>>> a.strip()
'aaaa  bbbb ccc    dddd   eee'
>>>

text string matching

libertycodervice — Wed, 04 Mar 2020 16:55:23 +0000

In Python programming, if you have a string you can do comparison or so called string matching.

Matches the beginning or end of the string is generally used in the matching file type or url

Simple text matching

A simple matching function is the general startwith() function.

>>> a = 'http://www.python.org'
>>> a.startswith ( 'http')
True

Note: This parameters which can be string, can also be a tuple. The example below passes a tuple as argument with two values.

>>> a = 'http://www.python.org'
>>> a.startswith (( 'http', 'ftp'))
True

As you may have guessed, you can also use the method endswith(), with a string or tuple.

>>> a = 'https://python.org'
>>> a.endswith(( '.org','.com' ))
True
>>>

While tuples are similar to lists, lists are not allowed as parameter.

The parameter can not be lists or dictionary. If you try a list or dictionary, then you get an error

>>> a = 'http://www.python.org'
>>> a.startswith ([ 'http', 'ftp'])
Traceback (most recent call last):
File "", line 1, in
a.startswith ([ 'http', 'ftp'])
TypeError: startswith first arg must be str or a tuple of str, not list
>>>

In fact, in addition to the above method, you can also use slices to achieve matching, but the code looks not so good:

>>> a = 'https://www.python.org'
>>> a[0:4] == 'https'
True
>>>

Regular expressions

Of course, we can also use regular expressions to do this, but on top of it a little difficult to understand a little.

>>> import re
>>> url = 'http://www.python.org'
>>> re.match ( 'http: | https: | ftp:', url)
0, 5), match = 'http:'>
>>> help (re.match)
Help on function match in module re:
      
match (pattern, string, flags = 0)
Try to apply the pattern at the start of the string, returning
a match object, or None if no match was found.
 
>>>

Popular python packages

libertycodervice — Tue, 03 Mar 2020 15:47:07 +0000

In this post I'll talk about some popular Python packages. These packages are used for a variety of purposes. You can install modules (packages) with the Python package manager pip.

Desktop software

For making a desktop applications (GUI graphical interface), you can use one of these packages. You can make any desktop software you want, including all the widgets (buttons, listbox, textbox etc)

Module	Description
PyQt	Make modern desktop software (QT development libraries for Python), (hello world tutorial)
wxpython	GUI programming framework in Python, with architecture similar to MFC
tkinter	Default module for building desktop software, tkinter tutorial

Web apps

To make a web app, you can use a web framework. Python is great for web development. You can easily deploy Python to the web.

Module	Description
Django	Open source web development framework that encourages rapid development and follows the MVC design
Flask	A simple Web framework, simple but powerful. (course)

Scientific computing

For scientific computing, there are all kinds of modules. If you want to do scientific computing, these are good:

Module	Description
Matplotlib	Use of third-party libraries like matlab implemented in Python, to draw some high-quality two-dimensional graphics mathematics
SciPy	Python-based matlab implementation, aimed at achieving all the features of matlab
NumPy	Python-based scientific computing third-party libraries, provides a matrix, linear algebra, Fourier transforms, etc. Solutions

Web scraping and others

For webpage scraping or data mining (web), there are some modules available. These modules let you grab a webpage and work with the data on that page.

Module	Description
BeautifulSoup	robust fault tolerance. Pages dealing with very powerful package
PyQuery	like jQuery, document processing

Other packages that are useful

Module	Description
MySQLdb	To connect MySQL database, example
PIL	Python-based image processing library, powerful, graphic file format support for a wide range of images
PyGame	Python-based multimedia and game development software development module
Sh Systems Management	sh lets you perform the same function execute shell commands like terminal
Pickle (cPickle)	export and import Python objects
Json	work with json data in python
Random	generate random number
Datetime	get date, hour, minute

Nested loops

libertycodervice — Tue, 03 Mar 2020 15:19:30 +0000

Python language allows loop embedded inside another. A loop repeats one or more lines of code.

In programming loops are usually for loops. If a loop is inside another loop, this is called a nested loop. You can make nested loops with a while loop.

Python for loop nested syntax:

    for iterating_var in sequence:
       for iterating_var in sequence:
          statements (s)
       statements (s)

Python while loop nested syntax:

    while expression:
       while expression:
          statement (s)
       statement (s)

You can be embedded in a loop inside the loop body, such as a while loop embedded in the for loop, and vice versa, you can embed in a for loop in a while loop.

Example

The following example uses a prime number between 2 and 100 nested loop output:

i = 2
while (i <100):
   j = 2
   while (j <= (i / j)):
      if not (i% j): break
      j = j + 1
   if (j > (i / j)):
      print(i, "is a prime number.")
   i = i + 1

print("Bye bye!")

Examples of the above output:

    2 is a prime number
    3 is a prime number
    5 is a prime number
    7 is a prime number
    11 is a prime number
    13 is a prime number
    17 is a prime number
    19 is a prime number
    23 is a prime number
    29 is a prime number
    31 is a prime number
    37 is a prime number
    41 is a prime number
    43 is a prime number
    47 is a prime number
    53 is a prime number
    59 is a prime number
    61 is a prime number
    67 is a prime number
    71 is a prime number
    73 is a prime number
    79 is a prime number
    83 is a prime number
    89 is a prime number
    97 is a prime number
    Bye bye!

More examples

Example: using nested loop to obtain a prime number less than 100

num = [];
i = 2
for i in range (2,100):
   j = 2
   for j in range(2, i):
      if (i% j == 0):
         break
   else:
      num.append(i)
print (num)

Example 2: nested loop to draw a pyramid

i = 1
# J = 1                                                                                                                                                                                  
while i <= 9:
   if i <= 5:
      print ("*" * i)

   elif i <= 9:
      j = i-2 * (i-5)
      print ("*" * j)
   i += 1
else:
   print ("")

This outputs a sideways pyramid:

*
**
***
****
*****
****
***
**
*

Python loop types

libertycodervice — Tue, 03 Mar 2020 15:06:11 +0000

In this article we introduce Python's loop, the program in general are executed sequentially. Sequentially means in sequential order (top down):

do A
do B
do C

In a variety of programming languages, control structures allows for more complex execution paths.

Loop allows us to perform a statement or group of statements repeatedly, the following is the general form in most programming languages loop statement:

Python is provided for and while loops (not do..while loop in Python):

Circulation type	Description
The while loop	determines the given condition is true. When the execution loop, or exit the loop.
For loop	Repeat statement
[Nested loop]	You can be nested for loop while loop body

The while loop in code:

>>> index = 0
>>> n = 10
>>> while index < n:
...     print(index)
...     index = index + 1
... 
0
1
2
3
4
5
6
7
8
9

The for loop:

>>> for i in range(1,3):
...     print(i)
... 
1
2

The nested for loop:

>>> 
>>> for x in range(1,10):
...     for y in range(1,10):
...         print(f"({x},{y})")

loop control statements

Loop control statements can change the order of statement execution. Python loop control supports the following statement:

Control statements	Description
Break Statement	Terminates the loop during the execution of statement block, and jump out of the entire cycle
Continue Statement	Terminates the current execution loop statement block, out of the cycle, the next cycle execution.
Pass sentence	pass an empty statement, in order to maintain the integrity of the program structure.

Python list (Lists)

libertycodervice — Tue, 03 Mar 2020 14:53:05 +0000

Python sequence is the most basic data structure. Each element in the sequence is assigned a number - its position, or index, the first index is 0, the second index is 1, and so on.

Python has a built-in type 6 sequences, but the most common are lists and tuples.

A sequence of operations can be carried out, including indexing, slicing, add, multiply, check members.

Further, Python already has built-in methods for sequence length, the maximum and minimum element. You may like this Python course

List

Python list is one of the most common types of data. It appears as a comma-separated values in square brackets.

Create a list, as long as the comma-separated data items using different brackets can be as follows:

>>> list1 = [ 'physics', 'chemistry', 1997, 2000]
>>> list2 = [1, 2, 3, 4, 5]
>>> list3 = [ "a", "b", "c", "d"]
>>>

Like string indices, list indices start at 0. The list can be intercepted, combinations and the like.

Access list values

The index is used to access the values in the list, as follows:

>>> list1 = [ 'physics', 'chemistry', 1997, 2000]
>>> list2 = [1, 2, 3, 4, 5]
>>> print(list1[0])
physics
>>> print(list2[0])
1
>>> print(list1[1:3])
['chemistry', 1997]
>>> print(list2[1:3])
[2, 3]
>>>

update list

You can modify or update data items in the list, you can also add a list item using append() method, as follows:

>>> list1 = [ 'physics', 'chemistry', 1997, 2000]
>>> list1[2] = 2010
>>> list1
['physics', 'chemistry', 2010, 2000]
>>> 
>>> list1.append("Tango")

Delete list elements

You can use the del statement to delete the list of elements, the following examples:

>>> list1 = [ 'physics', 'chemistry', 1997, 2000]
>>> list1
['physics', 'chemistry', 1997, 2000]
>>> del list1[1]
>>> list1
['physics', 1997, 2000]
>>>

Python script operator list

Similar lists of + and * operators string. + Number combination for the list, the list for repeatedly asterisk.

As follows:

Python expressions	result	Description
len ([1, 2, 3])	3	length
[1, 2, 3] + [4, 5, 6]	[1, 2, 3, 4, 5, 6]	composition
[ 'Hi!'] * 4	[ 'Hi!', 'Hi!', 'Hi!', 'Hi!']	Repeat
3 in [1, 2, 3]	True	element exists in the list
for x in [1, 2, 3]: print x,	1 2 3	iteration

Python interception list

Python list operation type string taken as follows:

    L = [ 'spam', 'Spam', 'SPAM!']

operating:

Python expressions	result	Description
L [2]	'SPAM!'	Third element in the list is read
L [-2]	'Spam'	penultimate read list element
L [1:]	[ 'Spam', '! SPAM']	taken from the second list element start

Python list of functions & methods

Python includes the following functions:

No.	function
1	cmp(list1, list2) Compare the two lists of elements
2	len(list) The number of list elements
3	max(list) Returns a list of elements of the maximum
4	min(list) Returns a list of elements of the minimum
5	list(seq) Convert a list of tuples

Python includes the following methods:

No.	Methods
1	list.append(obj) Add new object at the end of the list
2	list.count(obj) The number of times an element statistics appear in the list
3	list.extend(seq) A plurality of values in another sequence added at the end of the list of one-time (with a new list of the original extended list)
4	list.index(obj) A value index to find the location of the first match from the list
5	list.insert(index, obj) Inserting objects list
6	list.pop(obj = list [-1]) Removing one element in the list (default to the last element), and returns the value of the element
7	list.remove(obj) Remove the list a value of the first match
8	list.reverse() Reverse elements in the list
9	list.sort([func]) The original list is sorted

Python slice() function

libertycodervice — Mon, 02 Mar 2020 14:31:08 +0000

You can use the Pythons slice() function to create object slices. It is mainly used in the slicing operation in the function parameter.

You can use the slice() function for string slicing. The difference with default slicing [start:stop] is that it looks nice.

You can use it like slice(stop) or slice(start, stop [, step])``
The parameters are:

Start: Starting position
Stop: End position
Step: spacing

The function returns an object slice.

examples

The following example shows how to use the slice of:

>>> myslice = slice(5)
>>> myslice
slice(None, 5, None)
>>> 
>>> a = range(10)
>>> a
range(0, 10)
>>> a[myslice]
range(0, 5)
>>>

You can use the slice() method to to create a slice of a list. You can do this instead of [2:4]:

>>> x = ['a','b','c','d','e']
>>> x[slice(2,4)]
['c', 'd']

or a more beautiful way to write it:

>>> x = ['a','b','c','d','e']
>>> myslice = slice(2,4)
>>> x[myslice]
['c', 'd']
>>>

Python setter and getter function

libertycodervice — Mon, 02 Mar 2020 14:17:13 +0000

The Python setattr() function corresponds to the function getattr(). It set the attribute values of an object.

Python is an object orientated language, so you can have multiple objects in your applications run-time.

Objects (from classes) can have unique values. These values should be set using a getter and setter function. Python has the functions setattr() and getattr() for that.

The setattr() syntax is:

setattr (object, name, value)

with parameters:

Object: the object.
Name: string, object properties.
Value: property value.

examples

The following examples illustrate setattr() function using the method:

>>> class A:
...    foo = 1
... 
>>> obj1 = A()
>>> getattr(obj1, 'foo')
1
>>> setattr(obj1, 'foo', 6)
>>> getattr(obj1, 'foo')
6
>>>

Of course you can use properties without getter and setter, but that's a bad practice.

>>> obj1.foo
6
>>> obj1.foo = 7
>>>

If the property does not exist will create a new object property, and it will do property assignment:

>>> obj1.name = "Alice"
>>> getattr(obj1, 'name')
'Alice'
>>>

This modifies the existing object only, not the class the object is derived from.

>>> dir(A)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'foo']
>>>

But you should define all properties in the class, this is a best practice.

Related links:

Python next() function

libertycodervice — Mon, 02 Mar 2020 14:04:10 +0000

In this article we'll discuss the Python next() function. The Python next() function returns an iterator to the next item. If you have an iterable, it makes sense to use the next() function.

The next() syntax is:

next (iterator [, default])

Where the parameters are:

Iterator: iterables
Default: optional, used to set the default return value when there is no next element, if not set, and no next element is found it triggers the StopIteration exception.

The function return the current object in the iteratable.

examples

The following example shows how to use the next of:

#!/usr/bin/python
#-*- coding: UTF-8 -*-

# First get Iterator object:
it = iter ([1, 2, 3, 4, 5])

# Cycle:
while True:
    try:
        # Get the next value:
        x = next(it)
        print(x)
    except StopIteration:
        # Encounter StopIteration loop exits
        break

The output is:

It raises a StopIteration exception that you must catch, because otherwise the program stops abruptly (an exception is an 'error' that occurs while the program is running).

>>> 
>>> nums = [1,2,3,4,5,6]
>>> it = iter(nums)
>>> while True:
...     x = next(it)
...     print(x)
... 
1
2
3
4
5
6
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
StopIteration
>>>

So catch the StopIteration exception with a try-except block.

>>> nums = [1,2,3,4,5,6]
>>> it = iter(nums)
>>> while True:
...     try:
...         x = next(it)
...         print(x)
...     except StopIteration:
...         break
...

Related links:

Python id () function

libertycodervice — Mon, 02 Mar 2020 13:49:08 +0000

Python stores all of its data in the computers memory. The memory is a block that is divided into sub blocks, where each block has an address.

In Python, the id() function is used to obtain the memory address of the object.

Because Python is a high-level programming language, you usually don't need memory addresses. The value can be any variable, including a string.

The syntax of the id() function is:

    id([object])

The parameter it takes is the object. It returns the memory address of the object. Conceptually it's the same as a memory address in C, but depending on the implementation of Python it may not be.

Examples

The following examples show id to use:

>>> x = "python"
>>> id(x)
139738512291632
>>> y = "programming"
>>> id(y)
139738511022192
>>>

Python is quite smart with memory. If the value is the same, it will use the same memory address

>>> x = 10
>>> y = 10
>>> z = 10
>>> id(x)
9302464
>>> id(y)
9302464
>>> id(z)
9302464
>>>

These all point to the same location in memory, which is why their values are the same.

If the values are unique, the memory addresses will be different, because they point to different locations in memory:

>>> x = 10
>>> y = 11
>>> z = 12
>>> id(x)
9302464
>>> id(y)
9302496
>>> id(z)
9302528
>>>

Related links: