DEV Community: Python64

25 Python Examples

Python64 — Thu, 12 Mar 2020 16:16:11 +0000

This is a list of 25 examples for the Python programming language. They are common examples that you may use while programming.

The examples are code only and should be self-explicatory. If you are new to Python, you may like this book

Examples

Swap values between two variables

a = 4
b=6
a,b=b,a
print(a)
print(b)

Check if the given number is even

def is_even(num):
    return num % 2 == 0
is_even(12)

Split a multiline string into a list of lines

def split_lines(s):
    return s.split('\n')
split_lines('This is\n python\n data')

Find memory used by an object

import sys
print(sys.getsizeof(6))

Reverse a string

language = "python"
reversed_language = language[::-1]                                                              
print(reversed_language)

Print a string n times

def repeat(string, n):
    return (string * n)
repeat('python',5)

Check if a string is a palindrome

def palindrome(string):
    return string == string[::-1]
palindrome('python')
#palindrome mean a words or phrase or sequence that reads the same backwards as forwards

Combine a list of strings into a single string

print(','.join(strings))

Find the first element of a list

def head(list):
    return list[0]
print(head([1, 2, 3, 4, 5]))

Find elements that exist in either of the two lists

def union(a,b):
    return list(set(a + b))
union([1, 2, 3, 4, 5], [6, 2, 8, 1, 4])

Find all the unique elements present in a given list

def unique_elements(numbers):
    return list(set(numbers))
unique_elements([1, 2, 3, 2, 4])

Find the average of a list of numbers

def average(*args):
    return sum(args, 0.0) / len(args)
average(5, 8, 2)

Check if a list contains all unique values

def unique(list):
    if len(list)==len(set(list)):
        print("All elements are unique")
    else:
        print("List has duplicates")
unique([1,2,3,4,5])

Track frequency of elements in a list

from collections import Counter
list = [1, 2, 3, 2, 4, 3, 2, 3]
count = Counter(list)
print(count)

Find the most frequent element in a list

def most_frequent(list):
    return max(set(list), key = list.count)
numbers = [1, 2, 3, 2, 4, 3, 1, 3]
most_frequent(numbers)

Convert an angle from degrees to radians

import math
def degrees_to_radians(deg):
    return (deg * math.pi) / 180.0
degrees_to_radians(90)

Calculate time taken to execute a piece of code

import time
start_time = time.time()
a,b = 5,10
c = a+b
end_time = time.time()
time_taken = (end_time- start_time)*(10**6)
print("Time taken in micro_seconds:", time_taken)

Find gcd of a list of numbers

from functools import reduce
import math
def gcd(numbers):
    return reduce(math.gcd, numbers)
gcd([24,108,90])

Find unique characters in a string

string = "thisisapythonstring"   
unique = set(string)
new_string = ''.join(unique)
print(new_string)

Use lambda functions

x = lambda a, b, c : a + b + c
print(x(5, 10, 20))

Use map functions

def multiply(n): 
    return n * n 

list = (1, 3, 5) 
result = map(multiply, list) 
print(list(result))

Use filter functions

arr = [1, 2, 3, 4, 5]
arr = list(filter(lambda x : x%2 == 0, arr))
print (arr)

import pandas as pd
import numpy as ns

Use list comprehensions

numbers = [1, 2, 3]
squares = [number**2 for number in numbers]
print(squares)
[1, 4, 9]

Use slicing operator

def rotate(arr, d):
    return arr[d:] + arr[:d]

if __name__ == '__main__':
    arr = [1, 2, 3, 4, 5]
    arr = rotate(arr, 2)
    print (arr)
[3, 4, 5, 1, 2]

Use chained function call

def add(a, b):
    return a + b
def subtract(a, b):   
    return a - b
a, b = 5, 10
print((subtract if a > b else add)(a, b))

More Python:

graphical apps with tkinter

Python64 — Sun, 08 Mar 2020 22:28:48 +0000

You can use the tkinter module to create graphical applications and desktop applications with Python

For example, the Python desktop program below creates two buttons and a label. Tkinter has many widgets you can add to a window, labels and buttons being just one of them.

from tkinter import *

class OnOffButton(Button):
    def __init__(self,master=None,text=None):
        Button.__init__(self,master,text=text)
        self['command'] = self._onButtonClick

    def _onButtonClick(self):
        print('button clicked')


class App(Frame):
    def __init__(self,master=None):
        Frame.__init__(self,master)

        self.labelHello = Label(self,text="Hello World")
        self.labelHello['fg'] = "red"
        self.labelHello.grid()

        self.button1 = OnOffButton(self,text="Click Me")
        self.button1.grid()

        self.button2 = OnOffButton(self,text="Click me?")
        self.button2.grid()

def main():
    root = Tk()
    app = App(master=root)
    app.grid()
    root.mainloop()

if __name__ == '__main__':
    main()

In this example there are 3 widgets: a label and a button.
You can create a label by calling

Label(self,text="Hello World")

To create a button, call (the class is subclassed)

Button(self,text="Click me?")

So you can make all kinds of desktop apps with tkinter.
But you can create "graphical apps", similar to Paint. (source: canvas demo)

These are just simple examples. You can create much more complex graphical user interfaces, as shown in the image below:

Python input and output (basics)

Python64 — Sat, 07 Mar 2020 19:48:40 +0000

The Python input and output functions let you grab keyboard input, output to the screen, output to the files and more.

If you just start Python, it will open the interpreter (repl) and there you can type instructions.

In this article we cover some basics, you may or may not know.

String formatting

These are some basic things you can do in the python shell. From string output to calculations.

>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(1/7)
'0.14285714285714285'
>>> x = 10 * 3.25
>>> y = 200 * 200
>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
>>> print(s)
The value of x is 32.5, and y is 40000...

You can also use for loops in here, this repeats one or more lines of code. After a for loop, you must use four spaces as a way to show the begining and end of the block to repeat.

>>> for x in range(1, 11):
...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
...
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
 6  36  216
 7  49  343
 8  64  512
 9  81  729
10 100 1000

For loops can be interactive, by calling the input() method in each cycle or before the cycles start.

You can use zfill(n) to define the number of digits:

>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'

The format() function lets you add variables into your string (you can also use f-strings).

>>> print('Nevertheless {} coded in "{}!"'.format('Alice', 'Python'))
Nevertheless Alice coded in Python!

Dictionary

An interesting data structure supports in Python is the dictionary. This is a hashmap in computer science. Each key corresponds to a value. This is similar to a phonebook, which does a a map of name to number.

If you have a dictionary, you can output it like a table:

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
...     print('{0:10} ==> {1:10d}'.format(name, phone))
...
Jack       ==>       4098
Dcab       ==>       7678
Sjoerd     ==>       4127

For a more simple output you can use:

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678

Files

Did you know you work with real computer files in Python?
To open a file, you can use the functions open() and to read read().

A file can open for writing 'w', reading 'r', 'w+' writing and creating and more.

>>> f = open('/tmp/workfile', 'w')
>>> f.read()
'This is the entire file.\n'

You can also read a single line, instead of the whole file. In order for this to work, the file must already be opened.:

>>> f.readline()

To write a line into a file

>>> f.write('This is a test\n')

If you want to write an integer to a file:

>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)

Go functions

Python64 — Fri, 06 Mar 2020 23:02:09 +0000

A function is a basic code block that is used to perform a task.
The Go language has at least one main() function.

You can divide code by functions, and logically each function performs a specified task.

The function declaration tells the name of the compiler function, the return type, and the parameters.

Syntax

The function definition Go language function definition is in the following format:

func function_name( [parameter list] ) [return_types]{
}

Function definition resolution:

func: Function starts declaring function_name by func
function name: function name, and parameter list together form function signature.
parameter list: The parameter list, the parameter is like a placeholder, and when the function is called, you can pass the value(s) as parameter(s).

The parameter list specifies the parameter type, order, and number of arguments. The parameters are optional, that is, the function can also not contain arguments.
return_types: Return type, function returns a column value. Some features do not require a return value, such that return_types are not required.
Function body: A collection of codes defined by the function. The body is between the { and } symbols.

Examples

The following instance is the code of the max() function that passes in two integer arguments num1 and num2, and returns the maximum value of these two parameters:

/* function returns maximum value */
func max(num1, num2 int) int{
   result int

   if (num1 > num2) {
      result = num1
   } else {
      result = num2
   }
   return result 
}

When a function call creates a function, you define what the function needs to do and perform the specified task by calling the change function.

Call a function, pass arguments to the function, and return a value, such as:

package main

import "fmt"

func main() {
   var a int = 100
   var b int = 200
   var ret int

   ret = max(a, b)

   fmt.Printf( "Returns : %d\n", ret )
}

func max(num1, num2 int) int {
   var result int

   if (num1 > num2) {
      result = num1
   } else {
      result = num2
   }
   return result 
}

Reference:

Python lists

Python64 — Fri, 06 Mar 2020 21:49:46 +0000

In this aritcle you will learn about actions on a Python list. A list is not just defined, it's an interactive object to which you can add and remove items.

A list is just a collection of objects, which can be numbers, strings or other types of objects.

Some example lists:

a = [1,2,3]
b = ["hello","world","moon"]

insert an element into list

There is a previous method of appending an element to the list, which is only the last of the new elements added to the list.For example:

>>> all_users = ["alice","bob"]
>>> all_users.append("aaron")
>>> all_users
['alice', 'bob', 'aaron']
>>>

From this operation, it can be said that list can be changed at any time.

Similar to list.append(x), list.insert (i, x) can also add list elements. It is just that an element can be added anywhere.

list.insert(i, x)

This insert an item at a given position. The first argument is the index of the element at which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

Delete elements in list

The elements in the list can not only be added, but can also be deleted.There are two ways to delete a list element, which are:

list.remove(x)

Remove the first item from the list whose value is x. It is an error if
there is no such item.

list.pop([i])

Remove the item at the given position in the list, and return it. If no
index is specified, a.pop() removes and returns the last item in the list.

>>> all_users = ["alice","bob","aaron"]
>>> all_users.remove('bob')
>>> all_users
['alice', 'aaron']
>>>

Related links:

Install Python

Python64 — Thu, 05 Mar 2020 23:45:27 +0000

Python is installed by default on Mac OS X or Linux. But you may want to use Windows or install another version of Python. In this article we'll cover several things:

Python installation
Installation of Linux systems
windows system installation
Installation of Mac OS X systems
Install with ActivePython

Python installation

The things that you need to install are all in this page:
www.python.org/downloads/

Download the newest Python from there.

If you are new to Python, you may like the Python crash course

Installation of Linux systems

Python is cross-platform. It is already installed by default on most Linux distributions.

If you have to install it yourself, you can use your package manager to install it. On Ubuntu you can use apt-get:

sudo apt-get update
sudo apt-get install python3.6

From source

If you like to do things the hard way:

Download source code from the website
Decompression source pack

tar -zxvf Python-3.7.0.tgz

Compile

cd Python-3.7.0 ./configure -prefix=/usr/local make && make install

After the installation is good, enter the shell and type python and see the following:

$ python3
Python 3.7.5 (default, Nov 20 2019, 09:21:52) 
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Congratulations, the installation was successful.

Installation of windows systems

Go to (https://www.python.org/downloads/) find the windows installation package, download, for example, download this file: python-3.7.0.msi. Then it is a constant "next step" to complete the installation.

After installation, you need to check whether there is python in the environment variables.

If you don't know what is a windows environment variable and how to set it. Don't worry, search: "windows environment variable" on google.

The above is done. In cmd, enter python and you should see something like this:

$ python3
Python 3.7.5 (default, Nov 20 2019, 09:21:52) 
[GCC 9.2.1 20191008] on windows
Type "help", "copyright", "credits" or "license" for more information.
>>>

Installation of Mac OS X systems

In general, the Mac OS x systems have already installed a version of python, which can be used. If you want another, you can use brew.

But windows Python are definitely not installed.

In addition to the installation of the method mentioned above, there is also a more provincial approach, which is to install: ActivePython

Install with ActivePython

This ActivePython is a Python suite of several operating systems that includes a complete Python release, a Python programming IDE, and some Python. If interested visit their official website at : http://www.activestate.com/activepython

Use Source Installation

python is open-source and its source code is all online. You can https://www.python.org/ftp/python/, download the source code.

Then run these commands in the shell:

Access root
Download some version of python to the above address: wget https://www.python.org/ftp/python/3.7.0/Python-3.7.0.tgz
Extract:

tar xfz Python-3.7.0.tgz

Enter the directory:

cd Python-3.7.0

Configuration:

./configure

Run in the above folder: make, then run: make install
Good luck
Installation completed

OK! After you have installed it, you can start programming right away.

Related links:

Go environment

Python64 — Thu, 05 Mar 2020 20:18:14 +0000

Go is an open source programming language that makes it easy to build simple, reliable, and efficient software. Go supports the following systems:

Linux
FreeBSD
Mac OS X (also known as Darwin)
Windows

Download installation package is: https://golang.org/dl/.

Each system corresponding to the package name:

Operating system	package name
Windows	go1.4.windows-amd64.msi
Linux	go1.4.linux-amd64.tar.gz
Mac	go1.4.darwin-amd64-osx10.8.pkg
FreeBSD	go1.4.freebsd-amd64.tar.gz

UNIX / Linux / Mac OS X, and FreeBSD installation

The following describes the use of the source code in the UNIX / Linux / Mac OS X, and FreeBSD system installation method:

Download Source Package: go1.4.linux-amd64.tar.gz.

2, unzip the downloaded source package to /usr/ local directory.

tar -C /usr/ local -xzf go1.4.linux-amd64.tar.gz

3, add /usr/ local / go / bin directory to the PATH environment variable:

export PATH = $PATH:/usr/local/go/bin

Note: You can use the MAC system .pkg the end of the installation package by double-clicking to complete the installation, the installation directory /usr/local/go/down.

Windows system installation

Under Windows you can use .msi suffix (download list can be found in the file, such as go1.4.2.windows-amd64.msi) installation package to install.

By default, the .msi files will be installed in the c: \ directory Go. You can c: \ Go \ bin directory to the PATH environment variable. After adding a command window you need to restart to take effect.

installation testing

Create a working directory C: > Go_WorkSpace.

File name: test.go, code is as follows:

    package main
    
    import "fmt"
    
    func main () {
       fmt.Println ( "Hello, World!")
    }

Using the go command code above output results are as follows:

    C:\Go_WorkSpace> go run test.go
    
    Hello, World!

Related links:

Python tools

Python64 — Thu, 05 Mar 2020 19:47:04 +0000

In this article we'll discuss some python tools, these will make your development easier. Namely:

the python package manager pip
virtual environment
remote ssh.

Pip for package management

Python has many available packages or modules. These are existing code bases you can use to accelerate development. This is similar to libraries in C/C++.

To install module you need pip. To install pip, see this article.

For module installation, you can specify a version number

(sudo) pip install Django == 1.6.8

Upgrade package

(sudo) pip install bpython --upgrade

Install multiple packages

(sudo) pip install BeautifulSoup4 fabric virtualenv

Installation from the text, the text for the package name, one per line, you can specify a version number

(sudo) pip install -r requirements.txt

Delete a module

(sudo) pip uninstall xlrd

Export the currently installed package

pip freeze> requirements.txt

Virtualenv independent Python environmental management

virtualenv is a Python package to create a separate environment. Using virtualenv you won't have any module conflicts (as you may get when installing with sudo).

    # Installation:
    (sudo) pip install virtualenv virtualenvwrapper
    
    # .Bash_profile or modify .zshrc (if you use the words zsh), add the following statement
    export WORKON_HOME = $ HOME/.virtualenvs
    export PROJECT_HOME = $ HOME/workspace
    source /usr/local/bin/virtualenvwrapper.sh

mkvirtualenv ENV: create a running environment ENV
rmvirtualenv ENV: Delete operating environment ENV
mkproject mic: Creating runtime environment mic and mic project
mktmpenv: Create a temporary runtime environment
workon bsp: bsp work in the operating environment
lsvirtualenv: Lists the available operating environment
lssitepackages: Lists the current environment installed the package

Environment created is an independent, non-interfering, without the need to use sudo permissions pip to manage packages.

Documentation: https://virtualenvwrapper.readthedocs.org/en/latest/

Remote SSH

fabric: Fabric is a high level Python (2.7, 3.4+) library designed to execute shell commands remotely over SSH, yielding useful Python objects in return.

from fabric.api import *
    
# server list
env.hosts = [ 'user@server1', 'user2@server2']

def ls_home():
    with cd('/home/username/'):
        run('ls')

Official website: https://www.fabfile.org/
Documentation: https://docs.fabfile.org/

String split in Python

Python64 — Wed, 04 Mar 2020 16:30:14 +0000

In this article we simply talk about two methods of text parsing in Python. What we will do is given a string like

>>> line = 'aaa bbb ccc'

Split it into substrings, create strings based on this string.

Slice string

The first method is fragment by fragment. Define the recording offset, and then extract the desired string. [start:end]. Example:

>>> line = 'aaa bbb ccc'
>>> col1 = line[0: 3]
>>> col3 = line[8:]
>>> col1
'aaa'
>>> col3
'ccc'
>>>

However, this is undoable with a large string. Many developers use the .split () function.

Split function

The split() function turns a string into a list of strings. By default this function splits on spaces, meaning every word in a sentence will be a list item.

>>> line = 'aaa bbb ccc'
>>> a = line.split ( '')
>>> a
[ 'Aaa', 'bbb', 'ccc']
>>> a[0]
'Aaa'
>>> a[1]
'Bbb'
 >>> a[2]
'Ccc'
 >>>

You can split on character in the string, by setting the character in the split function. This can be a comma, a dash, a semicolon or even a dot (phrases).

>>> line = 'aaa, bbb, ccc'
>>> a = line.split(',')
>>> a
[ 'Aaa', 'bbb', 'ccc']
>>>

reverse string by character or word

Python64 — Wed, 04 Mar 2020 16:20:40 +0000

In this article, we use Python to reverse string character by character or word-by-word.

First we look at the string character by character reversal, because the python provides a very useful sections, so lets see

Reverse characters

You can reverse the characters in a string or a list of strings. The example below shows how to do that:

>>> a = 'abc edf degd'
>>> a [:: -1]
'dged fde cba'
>>>

You can also do this (same, but more clear):

>>> a = 'abcd abcd abcd'
>>> a [:: -1]
'dcba dcba dcba'
>>>

This can also be applied to a list of strings:

>>> a = 'abc abc abc'
>>> a[:: -1].split()
['cba', 'cba', 'cba']
>>>

Reverse word order

The method reverse may be used to reverse item order. In this example the string a is converted into a list of strings using the .split() method. Then the .reverse() method changes the order of the list.

>>> a = 'AAA BBB CCC'
>>> r = a.split()
>>> r
['AAA', 'BBB', 'CCC']
>>> r.reverse()
>>> r
['CCC', 'BBB', 'AAA']
>>>

The reverse() method can be applied on any list, so you can also use it for a list of numbers:

>>> a = [ 1,2,3,4,5,6 ]
>>> a.reverse()
>>> a
[6, 5, 4, 3, 2, 1]
>>>

while loop

Python64 — Wed, 04 Mar 2020 16:02:00 +0000

Python While Loops

You can use the Python programming to create loops. The most common loops in programming are the for loop, do while loops and the while loop.

So one of those loops is the while statement for executing a program loop. Under certain conditions, the same task is repeated.

The basic form:

    while judgment conditions:
        Execute the statement ......

Execute statement may be a single statement or a block. The condition can be any expression. If the determination condition is false, the cycle ends.

If you are new to Python, you may like this Python course

Performing the following flow chart:

Example:

count = 0
while (count <9): 
    print('The count is:', count)
    count = count + 1 
print("Good bye!")

Above the output code execution:

    The count is: 0
    The count is: 1
    The count is: 2
    The count is: 3
    The count is: 4
    The count is: 5
    The count is: 6
    The count is: 7
    The count is: 8
    Good bye!

There are two important statements that can be used in loops:

break is used to exit the loop
continue to skip this cycle,

# Continue and break usage
    
i = 1
while i <10: 
    i += 1 
    if i%2 > 0:
       continue
       print(i) # 2,4,6,8,10 output bis

i = 1
while 1: # 1 cycling conditions must be established
    print(i)
    i += 1
    if i > 10: # 10 out of the loop when i is greater than
        break

Infinite loop

If the conditional statement is always true, infinite loop will execute it, the following example:

var = 1
while var == 1: # This condition is always true, an infinite loop will execute down
    num = input( "Enter a number:")
    print("You entered:", num)

print("Good bye!")

Examples of the above output:

    Enter a number: 20
    You entered: 20
    Enter a number: 29
    You entered: 29
    Enter a number: 3
    You entered: 3
    Enter a number between: Traceback (most recent call last):
      File "test.py", line 5, in <module>
        num = input ( "Enter a number:")
    KeyboardInterrupt

That's because you enter num, but var stays the same each cycle.

Note: The above infinite loop you can use CTRL + C to interrupt the cycle.

recycled else statement

In python, you can use while ... else, as the example below:

count = 0
while count <5:
    print(count, "is less than 5")
    count = count + 1
else:
    print(count, "is not less than 5")

Examples of the above output is:

0 is less than 5
1 is less than 5
2 is less than 5
3 is less than 5
4 is less than 5
5 is not less than 5

python merge string

Python64 — Wed, 04 Mar 2020 15:36:37 +0000

In Python you can use variables. A variable lets you store some data in your program. A most basic variable would be

>>> a = 1
>>> b = 2

Variables can have types, the type above is a whole number (integer). One of the other variable types is the string, these contain character data. Either a character, a word or a phrase.

>>> s = "Hello"

If you want to merge strings, at first thought, of course, you can use + or + = connecting two strings

>>> a = 'a'
>>> b = 'b'
>>> c = a + b
>>> c
'ab'
>>>

If the entire program requires only two strings, that is no problem

However, if there exist a large number of strings, or even you need to loop stitching, this time there will be a performance problem

Therefore, I recommend using the .join method. If it is not the beginning of some string in place, you can use the temporary list, then join

E.g:

>>> a = 'a'
>>> b = 'b'
>>> c = a.join(b)
>>> c
'b'
>>>

To join two strings:

>>> x = " ".join((a,b))
>>> x
'a b'
>>>

To have no space use

>>> x = "".join((a,b))
>>> x
'ab'
>>>