f-string vs .format() vs % in Python: which should I use?
Use f-strings. The other two exist for backward compatibility and for one narrow case.
2 min Read
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How do I check if a list is empty in Python?
Just use if not items. Here is why that is preferred over comparing the length.
How do I copy a list in Python without changing the original?
Assignment does not copy. Here are the four ways that actually do — and when a shallow copy is not enough.
2 min ReadHow do I get the index while looping in Python?
enumerate() — and the start=1 argument almost everyone misses.
2 min ReadHow do I read a file in Python?
Use with open(...). Here is why, plus the encoding argument you should always pass.
2 min ReadHow do I reverse a list in Python?
Three ways, and the one difference between them that actually matters.
2 min ReadHow do I sort a dictionary by value in Python?
sorted() with a key function, then rebuild the dict. Two lines.
2 min ReadHow do I swap two variables in Python?
One line, no temporary variable. Here is what actually happens.
2 min ReadHow do I turn a list into a string in Python?
str.join() — and the one error it always throws first.
2 min ReadHow does try/except work in Python?
Catch what you can handle, name the exception, and never write a bare except.
2 min ReadWhat is a list comprehension and when should I use one?
A one-line way to build a list from another iterable — and the rule for when not to.
2 min ReadWhat is a Python virtual environment and do I need one?
Yes. It takes ten seconds and prevents the single most common Python setup problem.
2 min ReadWhat is the difference between == and is in Python?
One asks "same value?", the other asks "same object?". Mixing them up produces bugs that pass your tests.
2 min ReadWhat is the difference between a list and a tuple in Python?
Lists change, tuples do not — and that one difference drives everything else.
2 min ReadWhat is the difference between re.search and re.match?
match only looks at the start of the string. This is the number one regex mistake in Python.
2 min ReadWhen should I use dict.get() instead of square brackets?
Brackets crash on a missing key. get() returns None. Both are correct in different situations.
2 min ReadWhy does input() return a string in Python?
input() cannot know what you meant, so it always hands back text. Convert it the moment it arrives.
2 min ReadWhy does my Python default argument keep its old value?
The classic mutable default argument trap, and the one-line fix.
2 min Readecho vs print vs print_r vs var_dump in PHP
Two are for output, two are for debugging. Using the wrong one hides the bug you are hunting.
2 min ReadGET vs POST: which should my form use?
GET to read, POST to change. Thirty years on, that rule still decides it.
2 min ReadHow do I fix "headers already sent" in PHP?
Something printed before your header() call. Usually a stray blank line after a closing ?> tag.
2 min ReadHow do I fix "Undefined array key" in PHP?
You read a key that is not there. The null coalescing operator fixes it in one character.
2 min ReadHow do I format dates in PHP?
Use DateTimeImmutable, set a timezone explicitly, and store UTC.
2 min ReadHow do I handle a file upload safely in PHP?
Never trust the filename, never trust the reported type, and never store uploads where PHP can execute them.
2 min ReadHow do I hash a password in PHP?
password_hash and password_verify. Three lines, and nothing else.
2 min ReadHow do I prevent SQL injection in PHP?
Prepared statements. Not escaping, not filtering, not a blocklist of keywords.
2 min ReadHow do I work with JSON in PHP?
json_encode and json_decode, plus the flags that stop them failing silently.
2 min ReadHow do PHP sessions work?
A cookie holds an ID; the data stays on the server. Plus the one call that prevents session fixation.
2 min Readinclude vs require vs include_once in PHP
One warns and continues, the other stops. Choose based on whether the file is essential.
2 min ReadSingle vs double quotes in PHP: what is the difference?
Double quotes expand variables and escapes. Single quotes do not. That is the whole rule.
2 min ReadThe PHP array functions worth memorising
Twelve functions that replace most of the loops you were about to write.
2 min ReadWhat does htmlspecialchars() do and when do I need it?
It stops user input from becoming executable HTML. Use it on every echo of user data.
2 min ReadWhat is Composer and why does PHP need it?
Dependency manager and autoloader. It ends manual require statements.
2 min ReadWhat is the ?? operator in PHP?
Null coalescing — like the ternary, but it does not warn on missing keys.
2 min ReadWhy does == behave strangely in PHP?
Loose comparison converts types before comparing. Use === and the surprises stop.
2 min ReadWhy is my PHP page blank?
A white screen is a fatal error with error display turned off. Two lines make it visible.
2 min ReadBlocks, procs and lambdas in Ruby
All three are chunks of code you pass around. Two differences separate them.
2 min Readeach vs map in Ruby: what is the difference?
each is for doing, map is for making. The single most important distinction in Ruby.
2 min ReadHow do I handle nil safely in Ruby?
Safe navigation, ||=, and the difference between nil? and empty?
2 min ReadHow do ranges work in Ruby?
Two dots include the end, three exclude it — plus the places ranges show up beyond loops.
2 min ReadHow does string interpolation work in Ruby?
Double quotes and #{}. Plus format() for when you need columns.
2 min Readrequire vs require_relative in Ruby
One searches the load path, the other resolves against the current file.
2 min Readselect, reject, find and detect in Ruby
Keep matches, drop matches, or stop at the first one.
2 min ReadWhat does &. mean in Ruby?
The safe navigation operator — call a method only if the receiver is not nil.
2 min ReadWhat does attr_accessor do in Ruby?
It writes your getter and setter methods for you.
2 min ReadWhat is a gem and how does Bundler work?
Gems are Ruby packages. Bundler makes sure everyone gets the same versions.
2 min ReadWhat is a symbol in Ruby and when do I use one?
Symbols are immutable names. Use them as identifiers, strings as data.
2 min ReadWorking with hashes in Ruby
Access, iteration, defaults and the transform methods worth knowing.
2 min ReadArray vs List in C#: which should I use?
List for almost everything. Arrays for fixed-size data and interop.
2 min ReadHow do I fix a NullReferenceException in C#?
Something is null that you assumed was not. Here is how to find it and how to prevent the next one.
2 min ReadHow does async/await work in C#?
Free the thread while waiting. Plus the two rules that prevent deadlocks.
2 min ReadInterface vs abstract class in C#
Can-do versus is-a. And the practical rule: a class can implement many interfaces but inherit one base.
2 min ReadString formatting in C#
Interpolated strings, format specifiers, and why StringBuilder exists.
2 min ReadValue types vs reference types in C#
Structs copy, classes share. That difference explains most surprising C# behaviour.
2 min ReadWhat is LINQ and how do I use it?
Query syntax for collections. Ten methods cover almost everything.
2 min Readcond vs if in Racket
if takes exactly three parts. cond handles any number of branches.
2 min Readdefine vs let in Racket
define names things for a whole body; let names them for one expression.
2 min ReadHow do I write a recursive function in Racket?
A base case, and a step that makes the problem smaller. Always exactly those two.
2 min ReadHow do lists work in Racket?
A list is an item plus a smaller list. That recursive definition drives everything.
2 min ReadWhy does Racket put the operator first?
Prefix notation removes operator precedence entirely. That is the trade.
2 min ReadCSS position: relative vs absolute vs fixed
What each one is positioned against — and why absolute needs relative on the parent.
2 min ReadCSS transition vs animation: which do I need?
Transitions respond to a state change. Animations run on their own.
2 min ReadDebounce vs throttle: what is the difference?
Debounce waits for a pause. Throttle enforces a maximum rate.
2 min ReadFlexbox vs Grid: which should I use?
One dimension or two. That is the whole decision.
2 min ReadHow do I add dark mode to a website?
Define colours as custom properties, then redefine a handful of them. No component changes.
2 min ReadHow do I center a div in CSS?
Three lines with flexbox, two with grid. Both directions, no hacks.
2 min ReadHow do I fetch data from an API in JavaScript?
fetch plus await, and the response.ok check that everyone forgets.
2 min ReadHow do I fix "Cannot read properties of undefined"?
The most common JavaScript error. Read it carefully — it names the exact property.
2 min ReadHow do I make images load faster on my website?
Right format, right size, lazy loading, and always set width and height.
2 min ReadHow do I store data in the browser with localStorage?
Key-value strings that survive a refresh, plus the try/catch you actually need.
2 min ReadHow do I validate a form without JavaScript?
The browser does most of it. Here are the attributes, and where they stop.
2 min ReadHow does async/await work in JavaScript?
Syntax over promises that lets asynchronous code read top to bottom.
2 min Readlet vs const vs var in JavaScript
Use const by default, let when you must reassign, and var never.
2 min Readmap, filter and reduce in JavaScript
Transform, select, collapse. The three that replace most of your loops.
2 min Readrem vs em vs px: which unit should I use?
rem for almost everything, em for things that scale with their own text, px for hairlines.
2 min ReadShould I use == or === in JavaScript?
Use === always. There is exactly one defensible exception.
2 min ReadWhat are ::before and ::after used for?
Two extra elements per selector, free, with no markup. Plus the property that makes or breaks them.
2 min ReadWhat breakpoints should I use for responsive design?
Let the content decide. But if you want numbers, here are four that work.
2 min ReadWhat does "this" mean in JavaScript?
It depends on how the function was called — except for arrow functions, which is why they exist.
2 min ReadWhat does box-sizing: border-box do?
It makes width mean what you think it means. Put it in every stylesheet.
2 min ReadWhat is event delegation and why should I use it?
One listener on a parent instead of one per child — and it works for elements added later.
2 min ReadWhich HTML element should I use instead of a div?
Semantic elements give you structure, keyboard behaviour and screen-reader support for free.
2 min ReadWhy is my CSS not applying?
Usually specificity. Here is how to read it, and what to do instead of !important.
2 min ReadWhy is my z-index not working?
Either the element is not positioned, or it is trapped in a stacking context.
2 min ReadWhy is there extra space between my elements?
Margin collapsing, inline whitespace, or line-height. Three different causes, three different fixes.
2 min ReadArray vs linked list: when does it matter?
Arrays are fast to read, slow to insert. Linked lists are the reverse — and lose more often than textbooks suggest.
2 min Readgit merge vs rebase: which should I use?
Merge preserves history. Rebase rewrites it into a straight line. One rule decides.
2 min ReadHow do I resolve a git merge conflict?
Open the file, pick what should be there, remove the markers, commit.
2 min ReadHow do I undo the last commit in git?
Four commands, depending on whether you want to keep the changes and whether it is pushed.
2 min ReadHow does a hash table work?
A function turns a key into an array index. That is the whole trick.
2 min ReadRecursion vs iteration: which should I use?
Match the shape of the data. Recursive data wants recursive code.
2 min ReadStack vs queue: what is the difference?
Last in first out, versus first in first out. Which one you need tells you which algorithm you are writing.
2 min ReadThe terminal commands worth learning first
A dozen commands cover most day-to-day work.
2 min ReadWhat do HTTP status codes mean?
The first digit tells you almost everything.
2 min ReadWhat is a database index and when should I add one?
A lookup structure that turns a full table scan into a direct jump — at the cost of slower writes.
2 min ReadWhat is an API?
A contract for talking to someone else's program. Usually over HTTP, usually returning JSON.
2 min ReadWhat is Big-O notation?
How the work grows as the input grows. Six classes cover almost everything you will meet.
2 min ReadWhy does my text show weird characters like é?
Something wrote UTF-8 and something else read it as Latin-1. Fix the declaration, not the text.
2 min ReadWhy write tests, and what should I test first?
Tests let you change code without fear. Start with the logic that would be embarrassing to get wrong.
2 min ReadHow do I write binary search without off-by-one errors?
Pick one template and use it every time. Here is the one that works.
2 min ReadHow do I write FizzBuzz well?
Everyone can solve it. The variations reveal how you actually think.
2 min ReadReverse a linked list
Three pointers, one pass. The classic pointer-manipulation question.
2 min ReadTime complexity of common operations, by language
The lookup table worth internalising before an interview.
2 min ReadTwo Sum: how the hash map solution works
The problem that teaches the single most useful interview optimisation.
2 min ReadValid Parentheses: why a stack is the answer
Anything with nesting is a stack problem. This is the cleanest example.
2 min ReadWhat is the sliding window technique?
Turn a nested loop over subarrays into a single pass with two pointers.
2 min ReadWhat should I say out loud in a coding interview?
Silent coding gives the interviewer nothing to assess. Here is the sequence to narrate.
2 min Read