Learned to Build a simple pascal interpreter using python

Photo by Max Duzij on Unsplash

Recently I was going through the series of ‘Let’s Build A Simple Interpreter‘ by Ruslan Spivak which was given by Sayan as an assignment of summer training. Here you can see the whole series codes. I have gone through all the parts and added what I have learned in the README.md file.

In this series the main focus was to build an simple pascal interpreter by python which can execute simple pascal codes like mathematical operations and procedure calling. This series introduced me lots of thing like scanner, token, parser, interpreter and grammar, etc. At first I was thinking that I will not be able to complete the series properly as I didn’t know anything about interpreter and compiler but as move forward in this series things get cleared because everything was elaborated properly. Whenever I stuck at any point I search it in the internet and try to find the proper solution. For me in this series the most interesting part was the grammar of any programming language and how they help.

In this series I also learned how to write better commits so people can understand my code and also learned how to reset my commit to previous commit when I commit wrong things or made any bad commit message. Here you can see my commits.

Thank you 🙂

Fun with python requests module

I was reading this part of the pym book and thought that I should change this code a little bit so that it can do some better thing like instead of storing the content in a text file it stores it in a HTML file and it also checks if there is any same named file exists in the directory or not. Here is the code –

import os.path
import requests

def download(url):
    """
    Download the given url and saves it to the current directory
    :arg url: URL of the file to be downloaded.
    """
    req = requests.get(url)
    if req.status_code == 404:
        print('No such file found at %s' % url)
        return
    fileName = url.split('/')[-1].split('.')[0] + '.html'
    print(fileName)
    if os.path.isfile(fileName):
        print('Same file name already exist')
    else:
        with open(fileName, 'wb') as fobj:
            fobj.write(req.content)
        print('Download over')

if __name__ == "__main__":
    url = input("Enter a URL: ")
    download(url)

Above we are getting the content of the content of the url by requests.get(url) method. Then checking if that url is valid or not. If valid then parsing the url by split() method like first we are splitting it by “/” and taking the last value of the list and then splitting it again with “.” and taking the first value of the list. Then checking if there is no same name file exist and if there is no same name file then we are creating a file then writing the content in the file.
Thank you 🙂

Study, day 3

I have started learning python 3 from pym book by Kushal.

Read about following topics and solved problems.

  • Strings
    • Strip the string.Finding text.palindrome checking.Number of words.
  • Function
    • Defining function.Local and Global variable.keyword and keyword only arguments.Docstring.
  • File Handling.
    • Opening and closing a file.Reading data inside a file.Copying data from one file to another.Counting spaces, tabs and new lines in a file.
  • Exceptions
    • Various type of errors.
    • Handling exceptions.
    • Using finally keyword for cleaning.

Learning python3

Photo by Hitesh Choudhary on Unsplash

I started learning python back in 2017 and since then I am still learning it and using it and almost everyday I write some python code. Python is an interpreter based language, you can write the code directly in the interpreter or in a separate file(extension is .py) and run it.

Using the interpreter

Fire up your terminal and type python3. And you will see something like this below code snippet.

$ python3
Python 3.6.8 (default, Jan 14 2019, 11:02:34) 
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Now we are going to print “Hello World!” using the interpreter.

>>> print("Hello World!")
Hello World!

Using a source file

Create a file called helloWorld.py and enter the following text:

print("Hello World!")

We can run this file by python3 command.

$ python3 helloWorld.py 
Hello World!

Whitespaces and indentation

Language like c/c++ use pair of curly brackets to divide different identifiers. In the python it uses indentation for that. The whitespaces at the beginning of the line is known as indentation. If you give wrong whitespaces then it will give you errors. Python use 4 whitespaces for a single indentation. Below there is an example.

>>> a = "Aniruddha"
>>>  b = "Basak"
  File "<stdin>", line 1
    b = "Basak"
    ^
IndentationError: unexpected indent

Comments

Comment is a line in python that will start with # and the interpreter of python will just ignores the lines. You can write anything here to describe your code.

>>> # This is a comment
>>> # The next line will multiply two numbers
>>> a = 12 * 34
>>> print(a) # Printing the value of a

If your description is long then it is recommended that you use multiline comments. There is two options for that.

# This is option 1
# Multiline comment
"""
This is option 2
Multiline comment
"""

Modules

The reason why python is loved by so many people is it gives so many modules to work on almost everything you can imagine. Basically modules are python files that contains different functions classes and variable that you can reuse.

>>> import os
>>> print(os.name)
posix

Keywords and Identifiers

Below the following identifiers are main keywords of python. They must exactly types as it is.

False      class      finally    is         return
None       continue   for        lambda     try
True       def        from       nonlocal   while
and        del        global     not        with
as         elif       if         or         yield
assert     else       import     pass
break      except     in         raise

Variables and Datatypes

In python we don’t specify the type of the variable while declaring it.If we declare a = 1 then a will become an integer type and if we define b = "Hii" then b will become an string type variable.

>>> a = "Hi "
>>> b = "how you doing?"
>>> a + b
'Hi how you doing?'

Taking input from keyboard

We can take input from users while executing the program. We can use the input() function given by python for this. Let’s see an example of taking input from user.

number = int(input("Enter number: "))
print(number)

Here is the result –

$ python3 input.py 
Enter number: 25
25

Operators and Expressions

Python language supports the following types of operators.

  • Arithmetic Operators
  • Comparison (Relational) Operators
  • Assignment Operators
  • Logical Operators
  • Bitwise Operators
  • Membership Operators
  • Identity Operators

Arithmetic operator:

It is used to perform common mathematical operations.

>>> a = 2
>>> b = 4
>>> a + b
6
>>> a - b
-2
>>> a * b
8
>>> a / b
0.5
>>> a % b
2
>>> a ** b
16
>>> a // b
0

Comparison operator:

It is used for comparing two values and returns either True or False.

>>> a = 2
>>> b = 4
>>> a == b
False
>>> a != b
True
>>> a > b
False
>>> a < b
True
>>> a >= b
False
>>> a <= b
True

Assignment operator:

This operator is used to assign values to some variable.

>>> x = 5
>>> x += 5
>>> x -= 5
>>> x *= 5
>>> x /= 5

Logical operators:

Logical operators are the and, or, not operators.

>>> x = True
>>> y = False
>>> print('x and y is ', x and y)
x and y is  False
>>> print('x or y is', x or y)
x or y is True
>>> print('not x is', not x)
not x is False

Bitwise operators:

Bitwise operator works on bits and performs bit by bit operation. Assume if a = 60 and b = 13. Now in binary format they will be as follows –

>>> a = 60
>>> b = 30
>>> a & b
28
>>> a | b
62
>>> a ^ b
34
>>> ~a
-61
>>> a >> 2
15
>>> a << 2
240

Membership Operators:

Python’s membership operators test for membership in a sequence, such as strings, lists, or tuples. There are two membership operators as explained below −

a = 'Hello World'
print('H' in a)

Identity Operator:

Identity operators compare the memory locations of two objects. There are two Identity operators explained below −

>>> a = 5
>>> b = 5
>>> c = 10
>>> print(a is not b)
False
>>> print(a is b)
True
>>> print(a is c)
False

DGPLUG, more than a summer training

So, DGPLUG(Durgapur Linux Users Group) is a group where they provide free training to people who wanted to start contributing to FOSS. It is not only a group of Durgapur but it is a now international group. In this blog I am not going to tell you about how and what they teach in their summer training because I haven’t take their summer training yet this yearn I am going to take that but I am going to tell you something more than that like how a community helps to push you to the next level. Their motto is “শেখ এবং শেখাও” it’s a Bengali word meaning is “Learn and teach”.

So, on a regular Sunday I was checking Quora answers and suddenly one of Sayan’s answer written that he had started his journey of open source from DGPLUG and I pinged him on messenger that what is this DGPLUG. He said it is a Linux users group of Durgapur and they provide a summer training every year and you can join it on #dgplug on freenode. On the next day I joined the group and there was going some upstream project discussion and I was quite hesitated to say something and I privately messaged Kushal Das who is the founder of this group. I gave my introduction like who I am and which project I am contributing. He welcomed me and said you should tell this in the group so that everyone can know you. Then I started conversation with a “Hi” and gave my introduction and suddenly some folks welcomed me in this group and I was amazed that how this DGPLUG community is so good and welcoming to a new person and after that I never hesitated to ask any question in this group.

But it is not enough there is a lot more than that, after couple of weeks I made some good friends over there like Kushal, Nabarun, Sourabh and one who regularly comment on my blogs Jason. After that when I stuck any problem I ask their and I know their is somebody at least who is going to help to solve that problem. And now the most important thing what I have learnt from this group is “How to build better network”, “Not to private messaged anyone”, “Read everyday to increase your knowledge”, “Write what you have learnt”.

Thank you 🙂