Python Technical Interview – An Experience

As a freelancer one of the things that comes with getting a project/job is handling technical interviews. I have so far managed to convince the client with a work sample, test project …etc., This is literally the first time I sat for a full technical interview. And it did teach a few lessons. Let me document it for future use.

It started off with the basic of the language:

1. What is the difference between an iterable and an iterator?

Vincent Driessen provides a clear explanation of the difference with the examples here https://nvie.com/posts/iterators-vs-generators/

As an aside, he has a number of posts which are really great like his Git workflow model that I have used in my projects. Bookmark it

2. What is a Context Manager? What is its purpose? How is it different from a try…finally block? Why would you use one over another?

Context Manager are functions/classes that allow us to allocate and release resources as required. Used with the with keyword in code.

The difference between context manager and try..finally block is explained in technical detail here: https://stackoverflow.com/questions/26096435/is-python-with-statement-exactly-equivalent-to-a-try-except-finally-bloc

But a simpler more practical difference is given by Dan Bader: https://dbader.org/blog/python-context-managers-and-with-statement

3. Can you tell me some advantages of Python over other languages?

I rambled something like, it is is easier to read and write. The file structure (I should have said modules/packages) is great. Even modern iterations of Javascript are copying the import from syntax. Native implementation of a lot of things in standard library…etc.,et.,

But the thing my interviewer was looking for were the words “automatic garbage collection” because the next question was

4. How does Python handle memory?

Python has automated memory management and garbage collection.That is why we never worry about how much memory we are allocating like C’s malloc `calloc functions.

5. Do you know how Python does that? Do you know about GIL?

sheepish smiles and saying no’s ensued. I ran into an issue a few months back, I think maybe with a DB connection issue or something which led me on a rabbit hole that ended with GIL. I should have learnt it that day.

Anyway, here is the article about Python’s memory management. https://realpython.com/python-memory-management/

6. Have you worked on projects involving multi-threading? What do you know about multi-threading?

I hadn’t. Someday maybe I will.

7. Can you explain in detail the steps involved in a form submit to response cycle in detail?

https://developer.mozilla.org/en-US/docs/Learn/HTML/Forms/Sending_and_retrieving_form_data

8. How does the browser know where your server is when the information is submitted to a particular URL?

DNS servers – IP resolution

9. The server sends back text as a string how do you see colorful information in browser?

The text is converted into DOM elements which are rendered by the browsers rendering engine.

10. If a browser is showing unreadable character and question marks instead of displaying the information what could be the reason?

Document Encoding mismatch. The server might send the data encoded in Unicode UTF-8 and the browser might be decoding it as ASCII or LATIN-1 resulting in weird characters and question marks being rendered in the browser.

11. You said Unicode and UTF-8 what is the difference?

Unicode is the term used to describe the character set. If it is encoded with 8 bits it is called UTF-8, if encoded with 16 bits it is called UTF-16 etc.,

For deep dive into Unicode (a must): https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/

12. What kind of request does the browser make to a server? And what are the types of requests that can be made?

Browsers make a HTTP requests. The types are GET, POST, PUT, DELETE, HEAD, OPTIONS ..etc., (I think I said UPDATE instead of PUT, silly)

13. What is the difference between `==` and `===` in JavaScript?

StackOverflow: https://stackoverflow.com/questions/523643/difference-between-and-in-javascript

Some other questions, that were asked:
1. Do you know Docker? Have you used AWS?
2. Do you know Data Base schema design?
3. You have a SQL query that takes a long time to execute. How would you begin to make it faster? Do you know about Query optimisation and execution plans?

QGIS – Creating new column from existing using Python

Yesterday, I was working on the ward level parks map of Chennai I had to join a CSV data layer with the boundary polygon layer, but there was one issue while my CSV file has the ward numbers as integers (1,2,3..etc), the polygon layer had them as strings (Ward 1, Ward 2, Ward 3 …etc.,) So I was thinking, wouldn’t it be nice just to strip the word Ward and put it in a new column, so that I can make a join by matching the ward numbers. Turns out Python integration in QGIS is so good that, I did it without even searching the internet. Here is how.

  1. Open the Attribute table
  2. Open Field Calculator.
  3. Enter the “Output field name”
  4. Switch to “Function Editor”
  5. Click the [+] button to create a new function file.
  6. Changed the function name, parameter and return the value after stripping “Ward ” from the string. Read the docs given below the function editor to understand what’s going on the file.
QGIS Field Calculator
QGIS Field Calculator
from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def strip_ward(name, feature, parent ):
    return name.split(" ")[-1]

Now switch back to the Expression tab and call the function to calculate the new field

strip_ward.png

Click OK. Now the new field with the computed value would be created.

I had a simple use case, by one can use the power of Python to calculate anything from existing data and generate a new field based on it. I was really blown away by the level of Python integration in QGIS.

Python Tip #9 – sorting

Sorting is simplified in Python with sorted(). You can even sort with complex rules.

>>> strings = ['alice', 'bob', 'donald', 'cathy']
>>> sorted(strings)
['alice', 'bob', 'cathy', 'donald']

>>> sorted(strings, key=len)
['bob', 'alice', 'cathy', 'donald']

>>> def secondchar(word):
...    return word[1]

>>> sorted(strings, key=secondchar)
['cathy', 'alice', 'bob', 'donald']

Python Tip #8 – reducing looping by using dicts

In situations where you have a list of objects and have to retrieve then in random order, dictionaries can act as lookup tables.

users = list of class User 

# Get users one by one by looking up ids
user_1 = next((u for u in users if u.id == user_1_id), None)
user_2 = next((u for u in users if u.id == user_2_id), None)
...

# Simpler solution using lookup table
lookup = dict((u.id, user) for u in users)
user_1 = lookup[user_1_id]
user_2 = lookup[user_2_id]
...

This tip is not very obvious, hence this explanation:

user_1 = next((u for u in users if u.id == user_1_id), None)

This method employs a iterator looping through the list of users every time we have to find a user, which means we have to run this loop a hundred times. This poses a complexity of O(N2).

lookup = dict((u.id, user) for u in users)
user_1 = lookup[user_1_id]

This method on the other hand iterates through the users list one time and create a lookup table that we can again and again without having to iterate through the list every time. This reduces the complexity to O(N) which could theoretically lead up to 10 times faster execution of the program.

Python Tip #7 – getattr()

Sometimes we have to deal with external objects and their attributes. getattr() can save you at those times.

# Get the attribute name
name = obj.name  # AttributeError if name is not present

# Check if the attribute is present before fetching
try:
    name = obj.name
except AttributeError:
    name = "Guest"

# Simpler solution
name = obj.name if hasattr(obj, "name") else "Guest"

# Simplest Solution
name = getattr(obj, "name", "Guest")

Python Tip #6 – Merging Dictionaries

Merge or combine dictionaries

d1 = { "a": 1 }
d2 = { "b": 2 }

# Adding elements of one dictionary to another
d1.update(d2)  # d1 => { "a": 1, "b": 2 }

# Create a new dict with values from other dictionaries
d3 = { **d1, **d2 }  # d3 => { "a": 1, "b": 2 }
d4 = { **d3, "c": 3 }  # d4 => { "a": 1, "b": 2, "c": 3 }

** is the unpacking operator

Python Tip #5 – Get value from dict if key is present

Check for existence of a key in dictionary and retrieve its value if present.

dictionary = { "key": "value" }

# checking for the presence and key and getting the value
wanted = None
if "key" in dictionary:
    wanted = dictionary["key"]

# Simpler version
wanted = dictionary.get("key", None)