Notes I Wish I Had When I Started Learning Python
=================================================
Plus resources for learning data structures and algorithms in python at the bottom of this article!
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### Notes I Wish I Had When I Started Learning Python
#### Plus resources for learning data structures and algorithms in python at the bottom of this article!
### Basics
- **PEP8** : Python Enhancement Proposals, style-guide for Python.
- `print` is the equivalent of `console.log`.
- `#` is used to make comments in your code.
def foo():
"""
The foo function does many amazing things that you
should not question. Just accept that it exists and
use it with caution.
"""
secretThing()
- Python has a built in help function that let’s you see a description of the source code without having to navigate to it.
------------------------------------------------------------------------
### Numbers
- Python has three types of numbers:
- **Integer**
- Positive and Negative Counting Numbers.
- No Decimal Point
- Created by a literal non-decimal pt number or with the `int()` constructor.
print(3) # => 3 print(int(19)) # => 19 print(int()) # => 0
- Boolean is a subtype of integer in Python.
- **Floating Point Number**
- Decimal Numbers.
print(2.24) # => 2.24 print(2.) # => 2.0 print(float()) # => 0.0 print(27e-5) # => 0.00027
#### **Complex Numbers**
- Consist of a real part and imaginary part.
- The `i` is switched to a `j` in programming.
print(7j) # => 7j print(5.1+7.7j)) # => 5.1+7.7j print(complex(3, 5)) # => 3+5j print(complex(17)) # => 17+0j print(complex()) # => 0j
#### **Type Casting** : The process of converting one number to another.
# Using Float
print(17) # => 17
print(float(17)) # => 17.0
# Using Int
print(17.0) # => 17.0
print(int(17.0)) # => 17
# Using Str
print(str(17.0) + ' and ' + str(17)) # => 17.0 and 17
- The arithmetic operators are the same between JS and Python, with two additions:
- “\*\*” : Double asterisk for exponent.
- “//” : Integer Division.
- There are no spaces between math operations in Python.
- Integer Division gives the other part of the number from Module; it is a way to do round down numbers replacing `Math.floor()` in JS.
- There are no `++` and `--` in Python, the only shorthand operators are:
------------------------------------------------------------------------
### Strings
- Python uses both single and double quotes.
- You can escape strings like so `'Jodi asked, "What\'s up, Sam?"'`
- Multiline strings use triple quotes.
print('''My instructions are very long so to make them
more readable in the code I am putting them on
more than one line. I can even include "quotes"
of any kind because they won't get confused with
the end of the string!''')
- Use the `len()` function to get the length of a string.
print(len("Spaghetti")) # => 9
- Python uses `zero-based indexing`
- Python allows negative indexing (thank god!)
print("Spaghetti"[-1]) # => i print("Spaghetti"[-4]) # => e
- Python let’s you use ranges
print("Spaghetti"[1:4]) # => pag print("Spaghetti"[4:-1]) # => hett print("Spaghetti"[4:4]) # => (empty string)
- The end range is exclusive just like `slice` in JS.
# Shortcut to get from the beginning of a string to a certain index.
print("Spaghetti"[:4]) # => Spag
print("Spaghetti"[:-1]) # => Spaghett
# Shortcut to get from a certain index to the end of a string.
print("Spaghetti"[1:]) # => paghetti
print("Spaghetti"[-4:]) # => etti
- The `index` string function is the equiv. of `indexOf()` in JS
print("Spaghetti".index("h")) # => 4
print("Spaghetti".index("t")) # => 6
- The `count` function finds out how many times a substring appears in a string.
print("Spaghetti".count("h")) # => 1
print("Spaghetti".count("t")) # => 2
print("Spaghetti".count("s")) # => 0
print('''We choose to go to the moon in this decade and do the other things,
not because they are easy, but because they are hard, because that goal will
serve to organize and measure the best of our energies and skills, because that
challenge is one that we are willing to accept, one we are unwilling to
postpone, and one which we intend to win, and the others, too.
'''.count('the ')) # => 4
- You can use `+` to concatenate strings, just like in JS.
- You can also use “\*” to repeat strings or multiply strings.
- Use the `format()` function to use placeholders in a string to input values later on.
first_name = "Billy"
last_name = "Bob"
print('Your name is {0} {1}'.format(first_name, last_name)) # => Your name is Billy Bob
- Shorthand way to use format function is:
`print(f'Your name is {first_name} {last_name}')`
- Some useful string methods.
- Note that in JS `join` is used on an Array, in Python it is used on String.- There are also many handy testing methods.
------------------------------------------------------------------------
### Variables and Expressions
- **Duck-Typing** : Programming Style which avoids checking an object’s type to figure out what it can do.
- Duck Typing is the fundamental approach of Python.
- Assignment of a value automatically declares.
a = 7
b = 'Marbles'
print(a) # => 7
print(b) # => Marbles
- You can chain variable assignments to give multiple var names the same value.
- Use with caution as this is highly unreadable
count = max = min = 0
print(count) # => 0
print(max) # => 0
print(min) # => 0
- The value and type of a variable can be re-assigned at any time.
a = 17
print(a) # => 17
a = 'seventeen'
print(a) # => seventeen
- `NaN` does not exist in Python, but you can 'create' it like so:
`print(float("nan"))`
- Python replaces `null` with `none`.
- `none` is an object and can be directly assigned to a variable.
- Using none is a convenient way to check to see why an action may not be operating correctly in your program.
------------------------------------------------------------------------
### Boolean Data Type
- One of the biggest benefits of Python is that it reads more like English than JS does.# Logical AND
print(True and True) # => True
print(True and False) # => False
print(False and False) # => False
# Logical OR
print(True or True) # => True
print(True or False) # => True
print(False or False) # => False
# Logical NOT
print(not True) # => False
print(not False and True) # => True
print(not True or False) # => False
- By default, Python considers an object to be true UNLESS it is one of the following:
- Constant `None` or `False`
- Zero of any numeric type.
- Empty Sequence or Collection.
- `True` and `False` must be capitalized
------------------------------------------------------------------------
### Comparison Operators
- Python uses all the same equality operators as JS.
- In Python, equality operators are processed from left to right.
- Logical operators are processed in this order:
1. **NOT**
2. **AND**
3. **OR**
- Just like in JS, you can use `parentheses` to change the inherent order of operations.
- **Short Circuit** : Stopping a program when a `true` or `false` has been reached.
------------------------------------------------------------------------
### Identity vs Equality
print (2 == '2') # => False
print (2 is '2') # => False
print ("2" == '2') # => True
print ("2" is '2') # => True
# There is a distinction between the number types.
print (2 == 2.0) # => True
print (2 is 2.0) # => False
- In the Python community it is better to use `is` and `is not` over `==` or `!=`
------------------------------------------------------------------------
### If Statements
if name == 'Monica':
print('Hi, Monica.')
if name == 'Monica':
print('Hi, Monica.')
else:
print('Hello, stranger.')
if name == 'Monica':
print('Hi, Monica.')
elif age < 12:
print('You are not Monica, kiddo.')
elif age > 2000:
print('Unlike you, Monica is not an undead, immortal vampire.')
elif age > 100:
print('You are not Monica, grannie.')
- Remember the order of `elif` statements matter.
------------------------------------------------------------------------
### While Statements
spam = 0
while spam < 5:
print('Hello, world.')
spam = spam + 1
- `Break` statement also exists in Python.
spam = 0
while True:
print('Hello, world.')
spam = spam + 1
if spam >= 5:
break
- As are `continue` statements
spam = 0
while True:
print('Hello, world.')
spam = spam + 1
if spam < 5:
continue
break
------------------------------------------------------------------------
### Try/Except Statements
- Python equivalent to `try/catch`
a = 321
try:
print(len(a))
except:
print('Silently handle error here')
# Optionally include a correction to the issue
a = str(a)
print(len(a)
a = '321'
try:
print(len(a))
except:
print('Silently handle error here')
# Optionally include a correction to the issue
a = str(a)
print(len(a))
- You can name an error to give the output more specificity.
a = 100
b = 0
try:
c = a / b
except ZeroDivisionError:
c = None
print(c)
- You can also use the `pass` commmand to by pass a certain error.
a = 100
b = 0
try:
print(a / b)
except ZeroDivisionError:
pass
- The `pass` method won't allow you to bypass every single error so you can chain an exception series like so:
a = 100
# b = "5"
try:
print(a / b)
except ZeroDivisionError:
pass
except (TypeError, NameError):
print("ERROR!")
- You can use an `else` statement to end a chain of `except` statements.
# tuple of file names
files = ('one.txt', 'two.txt', 'three.txt')
# simple loop
for filename in files:
try:
# open the file in read mode
f = open(filename, 'r')
except OSError:
# handle the case where file does not exist or permission is denied
print('cannot open file', filename)
else:
# do stuff with the file object (f)
print(filename, 'opened successfully')
print('found', len(f.readlines()), 'lines')
f.close()
- `finally` is used at the end to clean up all actions under any circumstance.
def divide(x, y):
try:
result = x / y
except ZeroDivisionError:
print("Cannot divide by zero")
else:
print("Result is", result)
finally:
print("Finally...")
- Using duck typing to check to see if some value is able to use a certain method.
# Try a number - nothing will print out
a = 321
if hasattr(a, '__len__'):
print(len(a))
# Try a string - the length will print out (4 in this case)
b = "5555"
if hasattr(b, '__len__'):
print(len(b))
------------------------------------------------------------------------
### Pass
- Pass Keyword is required to write the JS equivalent of :
if (true) {
}
while (true) {}
if True:
pass
while True:
pass
------------------------------------------------------------------------
### Functions
- **Function** definition includes:
- The `def` keyword
- The name of the function
- A list of parameters enclosed in parentheses.
- A colon at the end of the line.
- One tab indentation for the code to run.
def printCopyright():
print("Copyright 2020. Me, myself and I. All rights reserved.")
- You can use default parameters just like in JS
def greeting(name, saying="Hello"):
print(saying, name)
greeting("Monica")
# Hello Monica
greeting("Barry", "Hey")
# Hey Barry
- Keep in mind, default parameters must always come after regular parameters.
# THIS IS BAD CODE AND WILL NOT RUN
def increment(delta=1, value):
return delta + value
- You can specify arguments by name without destructuring in Python.
def greeting(name, saying="Hello"):
print(saying, name)
# name has no default value, so just provide the value
# saying has a default value, so use a keyword argument
greeting("Monica", saying="Hi")
- The `lambda` keyword is used to create anonymous functions and are supposed to be `one-liners`.
`toUpper = lambda s: s.upper()`
------------------------------------------------------------------------
### Notes
### Formatted Strings
- Remember that in Python `join()` is called on a string with an array/list passed in as the argument.
shopping_list = ['bread','milk','eggs']
print(','.join(shopping_list))
- Python has a very powerful formatting engine.
- `format()` is also applied directly to strings.
# Comma Thousands Separator
print('{:,}'.format(1234567890))
'1,234,567,890'
# Date and Time
d = datetime.datetime(2020, 7, 4, 12, 15, 58)
print('{:%Y-%m-%d %H:%M:%S}'.format(d))
'2020-07-04 12:15:58'
# Percentage
points = 190
total = 220
print('Correct answers: {:.2%}'.format(points/total))
Correct answers: 86.36%
# Data Tables
width=8
print(' decimal hex binary')
print('-'*27)
for num in range(1,16):
for base in 'dXb':
print('{0:{width}{base}}'.format(num, base=base, width=width), end=' ')
print()
------------------------------------------------------------------------
### Getting Input from the Command Line
- Python runs synchronously, all programs and processes will stop when listening for a user input.
- The `input` function shows a prompt to a user and waits for them to type 'ENTER'.
------------------------------------------------------------------------
### Scripts vs Programs
- **Programming Script** : A set of code that runs in a linear fashion.
- The largest difference between scripts and programs is the level of complexity and purpose. Programs typically have many UI’s.
- Python can be used to display html, css, and JS.
- We will be using Python as an API (Application Programming Interface)
------------------------------------------------------------------------
### Structured Data
- **Sequence** : The most basic data structure in Python where the index determines the order.
- List
- Tuple
- Range
**Collections** : Unordered data structures, hashable values.
- Dictionaries
- Sets
- **Iterable** : Generic name for a sequence or collection; any object that can be iterated through.
- Can be mutable or immutable.
------------------------------------------------------------------------
### Built In Data Types
- **Lists** are the python equivalent of arrays.
empty_list = []
departments = ['HR','Development','Sales','Finance','IT','Customer Support']
# You can instantiate
specials = list()
# Test if a value is in a list.
print(1 in [1, 2, 3]) #> True
print(4 in [1, 2, 3]) #> False
- **Tuples** : Very similar to lists, but they are `immutable`
# Instantiated with parentheses
time_blocks = ('AM','PM')
# Sometimes instantiated without
colors = 'red','blue','green'
numbers = 1, 2, 3
# Tuple() built in can be used to convert other data into a tuple
tuple('abc') # returns ('a', 'b', 'c')
tuple([1,2,3]) # returns (1, 2, 3)
- Think of tuples as constant variables.
- **Ranges** : A list of numbers which can’t be changed; often used with `for` loops.
- Declared using one to three parameters.
1. **Start** : opt. default 0, first \# in sequence.
2. **Stop** : `required` next number past the last number in the sequence.
3. **Step** : opt. default 1, difference between each number in the sequence.
range(5) # [0, 1, 2, 3, 4]
range(1,5) # [1, 2, 3, 4]
range(0, 25, 5) # [0, 5, 10, 15, 20]
range(0) # [ ]
for let (i = 0; i < 5; i++)
for let (i = 1; i < 5; i++)
for let (i = 0; i < 25; i+=5)
for let(i = 0; i = 0; i++)
- Keep in mind that `stop` is not included in the range.
- `Dictionaries` : Mappable collection where a hashable value is used as a key to ref. an object stored in the dictionary.
- Mutable.
a = {'one':1, 'two':2, 'three':3}
b = dict(one=1, two=2, three=3)
c = dict([('two', 2), ('one', 1), ('three', 3)])
> *a, b, and c are all equal*
- Declared with curly braces of the built in `dict()`
- Benefit of dictionaries in Python is that it doesn’t matter how it is defined, if the keys and values are the same the dictionaries are considered equal.
- Use the `in` operator to see if a key exists in a dictionary.
#### **Sets** : Unordered collection of distinct objects; objects that **need** to be hashable.
- Always be unique, duplicate items are auto dropped from the set.
- **Common Uses**:
- Removing Duplicates
- Membership Testing
- Mathematical Operators: Intersection, Union, Difference, Symmetric Difference.
- Standard Set is mutable, Python has a immutable version called `frozenset`.
- Sets created by putting comma seperated values inside braces:
school_bag = {'book','paper','pencil','pencil','book','book','book','eraser'}
print(school_bag)
# Also can use set constructor to automatically put it into a set.
letters = set('abracadabra')
print(letters)
------------------------------------------------------------------------
### Built-In Functions
**Functions using iterables**
- **filter(function, iterable)** : creates new iterable of the same type which includes each item for which the function returns true.
- **map(function, iterable)** : creates new iterable of the same type which includes the result of calling the function on every item of the iterable.
- **sorted(iterable, key=None, reverse=False)** : creates a new sorted list from the items in the iterable.
- Output is always a `list`
- `key`: opt function which coverts and item to a value to be compared.
- `reverse`: optional boolean.
- **enumerate(iterable, start=0)** : starts with a sequence and converts it to a series of tuples
quarters = ['First', 'Second', 'Third', 'Fourth']
print(enumerate(quarters))
print(enumerate(quarters, start=1))
# (0, 'First'), (1, 'Second'), (2, 'Third'), (3, 'Fourth')
# (1, 'First'), (2, 'Second'), (3, 'Third'), (4, 'Fourth')
- **zip(\*iterables)** : creates a zip object filled with tuples that combine 1 to 1 the items in each provided iterable.
**Functions that analyze iterables**
- **len(iterable)** : returns the count of the number of items.
- **max(\*args, key=None)** : returns the largest of two or more arguments.
- **max(iterable, key=None)** : returns the largest item in the iterable.
- `key` optional function which converts an item to a value to be compared.
- **min** works the same way as `max`
- **sum(iterable)** : used with a list of numbers to generate the total.
- There is a faster way to concatenate an array of strings into one string, so do not use sum for that.
- **any(iterable)** : returns True if any items in the iterable are true.
- **all(iterable)** : returns True is all items in the iterable are true.
**Working with dictionaries**
- **dir(dictionary)** : returns the list of keys in the dictionary.
**Working with sets**
- **Union** : The pipe | operator or `union(*sets)` function can be used to produce a new set which is a combination of all elements in the provided set.
a = {1, 2, 3}
b = {2, 4, 6}
print(a | b) # => {1, 2, 3, 4, 6}
- **Intersection** : The & operator ca be used to produce a new set of only the elements that appear in all sets.
a = {1, 2, 3}
b = {2, 4, 6}
print(a & b) # => {2}
- **Difference** : The — operator can be used to produce a new set of only the elements that appear in the first set and NOT the others.
- **Symmetric Difference** : The ^ operator can be used to produce a new set of only the elements that appear in exactly one set and not in both.
a = {1, 2, 3}
b = {2, 4, 6}
print(a - b) # => {1, 3}
print(b - a) # => {4, 6}
print(a ^ b) # => {1, 3, 4, 6}
------------------------------------------------------------------------
### For Statements
- In python, there is only one for loop.
- Always Includes:
- The `for` keyword
- A variable name
- The `in` keyword
- An iterable of some kid
- A colon
- On the next line, an indented block of code called the `for` clause.
- You can use `break` and `continue` statements inside for loops as well.
- You can use the range function as the iterable for the `for` loop.
print('My name is')
for i in range(5):
print('Carlita Cinco (' + str(i) + ')')
total = 0
for num in range(101):
total += num
print(total)
- Looping over a list in Python
for c in ['a', 'b', 'c']:
print(c)
lst = [0, 1, 2, 3]
for i in lst:
print(i)
- Common technique is to use the len() on a pre-defined list with a for loop to iterate over the indices of the list.
supplies = ['pens', 'staplers', 'flame-throwers', 'binders']
for i in range(len(supplies)):
print('Index ' + str(i) + ' in supplies is: ' + supplies[i])
- You can loop and destructure at the same time.
l = [[1, 2], [3, 4], [5, 6]]
for a, b in l:
print(a, ', ', b)
# Prints 1, 2
# Prints 3, 4
# Prints 5, 6
- You can use `values()` and `keys()` to loop over dictionaries.
spam = {'color': 'red', 'age': 42}
for v in spam.values():
print(v)
# Prints red
# Prints 42
for k in spam.keys():
print(k)
# Prints color
# Prints age
- For loops can also iterate over both keys and values.
# Getting tuples
for i in spam.items():
print(i)
# Prints ('color', 'red')
# Prints ('age', 42)
# Destructuring to values
for k, v in spam.items():
print('Key: ' + k + ' Value: ' + str(v))
# Prints Key: age Value: 42
# Prints Key: color Value: red
- Looping over string
for c in "abcdefg":
print(c)
------------------------------------------------------------------------
### More On Functions
- **Variable-length positional arguments** : (\*args)
def add(a, b, *args):
total = a + b;
for n in args:
total += n
return total
add(1, 2) # Returns 3
add(2, 3, 4, 5) # Returns 14
- **keyword arguments** : (\*kwargs)
def print_names_and_countries(greeting, **kwargs):
for k, v in kwargs.items():
print(greeting, k, "from", v)
print_names_and_countries("Hi",
Monica="Sweden",
Charles="British Virgin Islands",
Carlo="Portugal")
# Prints
# Hi Monica from Sweden
# Hi Charles from British Virgin Islands
# Hi Carlo from Portugal
- When you order arguments within a function or function call, the args need to occur in a particular order:
1. formal positional args.
2. \*args
3. keyword args with default values
4. \*\*kwargs
def example(arg_1, arg_2, *args, **kwargs):
pass
def example2(arg_1, arg_2, *args, kw_1="shark", kw_2="blowfish", **kwargs):
pass
------------------------------------------------------------------------
### Importing in Python
- Modules are similar to packages in Node.js
- Come in different types: Built-In, Third-Party, Custom.
- All loaded using `import` statements.
**Terms**
- **module** : Python code in a separate file.
- **package** : Path to a directory that contains modules.
- \***\*init**.py\*\* : Default file for a package.
- **submodule** : Another file in a module’s folder.
- **function** : Function in a module.
- A module can be any file but it is usually created by placing a special file `__init__.py` into a folder.
- Try to avoid importing with wildcards in Python.
- Use multiple lines for clarity when importing.
from urllib.request import (
HTTPDefaultErrorHandler as ErrorHandler,
HTTPRedirectHandler as RedirectHandler,
Request,
pathname2url,
url2pathname,
urlopen,
)
------------------------------------------------------------------------
### Watching Out for Python 2
- Python 3 removed `<>` and only uses `!=`
- `format()` was introduced with P3
- All strings in P3 are unicode and encoded.
- `md5` was removed.
- `ConfigParser` was renamed to `configparser`
- `sets` were killed in favor of `set()` class.
- `print` was a statement in P2, but is a function in P3.
------------------------------------------------------------------------
### Classes In Python
- Classes are a way of combining information and behavior.
- Classes are blueprints to make objects.
class AngryBird {
constructor() {
this.x = 0;
this.y = 0;
}
}
class AngryBird:
def __init__(self):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self.x = 0
self.y = 0
- Both JS and PY use the `class` keyword to declare classes.
- `constructor` == `__init__`
- `this` == `self`
bird = AngryBird()
print(bird.x, bird.y) #> 0 0
class AngryBird:
def __init__(self):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self.x = 0
self.y = 0
def move_up_by(self, delta):
self.y += delta
- Note how you do not need to define `self` it is already bound to the class.
- It is good practice to write a comment at the beginning of your class, describing the class.
**Dunder Methods**
- Double Underscore Methods, special built in functions that PY uses in certain ways.
- i.e. `__init__()` lets you make sure all relevant attributes are set to their proper values when an object is created from the class.
- The `self` keyword refers to the current object that you are working with.
- Method is a function that is part of a class.
class AngryBird:
def __init__(self):
self.x = 0
self.y = 0
def move_up_by(self, delta):
self.y += delta
bird = AngryBird()
print(bird)
print(bird.y)
bird.move_up_by(5)
print(bird.y)
- *Use one leading underscore only for non-public methods and instance variables*
class AngryBird:
def __init__(self, x=0, y=0):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self._x = x
self._y = y
def move_up_by(self, delta):
self._y += delta
def get_x(self):
return self._x
def get_y(self):
return self._y
- *All instance variables should be considered non-public*
- \***\*slots\*\*** : Dunder class variable used to reserve memory for the instance variables that you know will you will use.
class AngryBird:
__slots__ = ['_x', '_y']
def __init__(self, x=0, y=0):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self._x = x
self._y = y
def move_up_by(self, delta):
self._y += delta
def get_x(self):
return self._x
def get_y(self):
return self._y
- You can use `__repr__()` to override the behavior of printing out a class in a verbose manner.
class AngryBird:
__slots__ = ['_x', '_y']
def __init__(self, x=0, y=0):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self._x = x
self._y = y
def move_up_by(self, delta):
self._y += delta
def get_x(self):
return self._x
def get_y(self):
return self._y
def __repr__(self):
return f""
------------------------------------------------------------------------
### Properties for Classes
- Getters and Setters are used in object-oriented programming to add validation logic around getting and setting a value.
**Getters**
bird = AngryBird()
print(bird.get_x(), bird.get_y())
- Getting the x and y values of our class can get very cumbersome.
- **Decorators** : Allow us to change the way methods get invoked.
- Always start with the @ symbol.
- Can be applied to methods, classes, and parameters.
- Built in decorator named `property` that you can apply to a method to make it readable.
@property
def x(self):
return self._x
@property
def y(self):
return self._y
bird = AngryBird()
print(bird.x, bird.y)
**Setters**
class AngryBird:
def __init__(self, x=0, y=0):
"""
Construct a new AngryBird by setting its position to (0, 0).
"""
self._x = x
self._y = y
def move_up_by(self, delta):
self._y += delta
@property
def x(self):
return self._x
@x.setter
def x(self, value):
if value < 0:
value = 0
self._x = value
@property
def y(self):
return self._y
@y.setter
def y(self, value):
if value < 0:
value = 0
self._y = value
------------------------------------------------------------------------
### List Comprehensions
- List comprehensions are the equivalent of wrapped up filter namp array methods while also allowing nested loops.
- `new_list = [expression for member in iterable]`
- **expression** : member itself, a call to a methd, or any other valid expression that returns a value.
- **member** : object or value in the list or iterable.
- **iterable** : iterable.
`new_list = [expression for member in iterable (if conditional)]`
- Adding a conditional into a list comprehension.
sentence = 'Mary, Mary, quite contrary, how does your garden grow?'
def is_consonant(letter):
vowels = "aeiou"
return letter.isalpha() and letter.lower() not in vowels
consonants = [i for i in sentence if is_consonant(i)]
print(consonants)
# Prints ['M', 'r', 'y', 'M', 'r', 'y', 'q', 't', 'c',
# 'n', 't', 'r', 'r', 'y', 'h', 'w', 'd', 's', 'y',
# 'r', 'g', 'r', 'd', 'n', 'g', 'r', 'w']
**When to not use list comprehensions**
- List comprehensions may make your code run more slowly or use more memory.
- You can use nest lists to create matrices.
matrix = [[i for i in range(5)] for _ in range(6)]
print(matrix)
# Prints
# [
# [0, 1, 2, 3, 4],
# [0, 1, 2, 3, 4],
# [0, 1, 2, 3, 4],
# [0, 1, 2, 3, 4],
# [0, 1, 2, 3, 4],
# [0, 1, 2, 3, 4]
# ]
------------------------------------------------------------------------
### My Blog:
Web-Dev-Hub Memoization, Tabulation, and Sorting Algorithms by Example Why is looking at runtime not a reliable method of…master--bgoonz-blog.netlify.app
### Python Data Structures & Algorithms Resources:
- The Framework for Learning Algorithms and intense problem solving exercises
- Algs4: Recommended book for Learning Algorithms and Data Structures
- An analysis of Dynamic Programming
- Dynamic Programming Q&A — What is Optimal Substructure
- The Framework for Backtracking Algorithm
- Binary Search in Detail: I wrote a Poem
- The Sliding Window Technique
- Difference Between Process and Thread in Linux
- Some Good Online Practice Platforms
- Dynamic Programming in Details
- Dynamic Programming Q&A — What is Optimal Substructure
- Classic DP: Longest Common Subsequence
- Classic DP: Edit Distance
- Classic DP: Super Egg
- Classic DP: Super Egg (Advanced Solution)
- The Strategies of Subsequence Problem
- Classic DP: Game Problems
- Greedy: Interval Scheduling
- KMP Algorithm In Detail
- A solution to all Buy Time to Buy and Sell Stock Problems
- A solution to all House Robber Problems
- 4 Keys Keyboard
- Regular Expression
- Longest Increasing Subsequence
- The Framework for Learning Algorithms and intense problem solving exercises
- Algs4: Recommended book for Learning Algorithms and Data Structures
- Binary Heap and Priority Queue
- LRU Cache Strategy in Detail
- Collections of Binary Search Operations
- Special Data Structure: Monotonic Stack
- Special Data Structure: Monotonic Stack
- Design Twitter
- Reverse Part of Linked List via Recursion
- Queue Implement Stack/Stack implement Queue
- My Way to Learn Algorithm
- The Framework of Backtracking Algorithm
- Binary Search in Detail
- Backtracking Solve Subset/Permutation/Combination
- Diving into the technical parts of Double Pointers
- Sliding Window Technique
- The Core Concept of TwoSum Problems
- Common Bit Manipulations
- Breaking down a Complicated Problem: Implement a Calculator
- Pancake Sorting Algorithm
- Prefix Sum: Intro and Concept
- String Multiplication
- FloodFill Algorithm in Detail
- Interval Scheduling: Interval Merging
- Interval Scheduling: Intersections of Intervals
- Russian Doll Envelopes Problem
- A collection of counter-intuitive Probability Problems
- Shuffle Algorithm
- Recursion In Detail
- How to Implement LRU Cache
- How to Find Prime Number Efficiently
- How to Calculate Minimium Edit Distance
- How to use Binary Search
- How to efficiently solve Trapping Rain Water Problem
- How to Remove Duplicates From Sorted Array
- How to Find Longest Palindromic Substring
- How to Reverse Linked List in K Group
- How to Check the Validation of Parenthesis
- How to Find Missing Element
- How to Find Duplicates and Missing Elements
- How to Check Palindromic LinkedList
- How to Pick Elements From an Infinite Arbitrary Sequence
- How to Schedule Seats for Students
- Union-Find Algorithm in Detail
- Union-Find Application
- Problems that can be solved in one line
- Find Subsequence With Binary Search
- Difference Between Process and Thread in Linux
- You Must Know About Linux Shell
- You Must Know About Cookie and Session
- Cryptology Algorithm
- Some Good Online Practice Platforms
### Algorithms:
- 100 days of algorithms
- Algorithms — Solved algorithms and data structures problems in many languages.
- Algorithms by Jeff Erickson (Code) (HN)
- Top algos/DS to learn
- Some neat algorithms
- Mathematical Proof of Algorithm Correctness and Efficiency (2019)
- Algorithm Visualizer — Interactive online platform that visualizes algorithms from code.
- Algorithms for Optimization book
- Collaborative book on algorithms (Code)
- Algorithms in C by Robert Sedgewick
- Algorithm Design Manual
- MIT Introduction to Algorithms course (2011)
- How to implement an algorithm from a scientific paper (2012)
- Quadsort — Stable non-recursive merge sort named quadsort.
- System design algorithms — Algorithms you should know before system design.
- Algorithms Design book
- Think Complexity
- All Algorithms implemented in Rust
- Solutions to Introduction to Algorithms book (Code)
- Maze Algorithms (2011) (HN)
- Algorithmic Design Paradigms book (Code)
- Words and buttons Online Blog (Code)
- Algorithms animated
- Cache Oblivious Algorithms (2020) (HN)
- You could have invented fractional cascading (2012)
- Guide to learning algorithms through LeetCode (Code) (HN)
- How hard is unshuffling a string?
- Optimization Algorithms on Matrix Manifolds
- Problem Solving with Algorithms and Data Structures (HN) (PDF)
- Algorithms implemented in Python
- Algorithms implemented in JavaScript
- Algorithms & Data Structures in Java
- Wolfsort — Stable adaptive hybrid radix / merge sort.
- Evolutionary Computation Bestiary — Bestiary of evolutionary, swarm and other metaphor-based algorithms.
- Elements of Programming book — Decomposing programs into a system of algorithmic components. (Review) (HN) (Lobsters)
- Competitive Programming Algorithms
- CPP/C — C/C++ algorithms/DS problems.
- How to design an algorithm (2018)
- CSE 373 — Introduction to Algorithms, by Steven Skiena (2020)
- Computer Algorithms II course (2020)
- Improving Binary Search by Guessing (2019)
- The case for a learned sorting algorithm (2020) (HN)
- Elementary Algorithms — Introduces elementary algorithms and data structures. Includes side-by-side comparisons of purely functional realization and their imperative counterpart.
- Combinatorics Algorithms for Coding Interviews (2018)
- Algorithms written in different programming languages (Web)
- Solving the Sequence Alignment problem in Python (2020)
- The Sound of Sorting — Visualization and “Audibilization” of Sorting Algorithms. (Web)
- Miniselect: Practical and Generic Selection Algorithms (2020)
- The Slowest Quicksort (2019)
- Functional Algorithm Design (2020)
- Algorithms To Live By — Book Notes
- Numerical Algorithms (2015)
- Using approximate nearest neighbor search in real world applications (2020)
- In search of the fastest concurrent Union-Find algorithm (2019)
- Computer Science 521 Advanced Algorithm Design
### Data Structures:
- Data Structures and Algorithms implementation in Go
- Which algorithms/data structures should I “recognize” and know by name?
- Dictionary of Algorithms and Data Structures
- Phil’s Data Structure Zoo
- The Periodic Table of Data Structures (HN)
- Data Structure Visualizations (HN)
- Data structures to name-drop when you want to sound smart in an interview
- On lists, cache, algorithms, and microarchitecture (2019)
- Topics in Advanced Data Structures (2019) (HN)
- CS166 Advanced DS Course (2019)
- Advanced Data Structures (2017) (HN)
- Write a hash table in C
- Python Data Structures and Algorithms
- HAMTs from Scratch (2018)
- JavaScript Data Structures and Algorithms
- Implementing a Key-Value Store series
- Open Data Structures — Provide a high-quality open content data structures textbook that is both mathematically rigorous and provides complete implementations. (Code)
- A new analysis of the false positive rate of a Bloom filter (2009)
- Ideal Hash Trees
- RRB-Trees: Efficient Immutable Vectors
- Some data structures and algorithms written in OCaml
- Let’s Invent B(+)-Trees (HN)
- Anna — Low-latency, cloud-native KVS.
- Persistent data structures thanks to recursive type aliases (2019)
- Log-Structured Merge-Trees (2020)
- Bloom Filters for the Perplexed (2017)
- Understanding Bloom Filters (2020)
- Dense vs. Sparse Indexes (2020)
- Data Structures and Algorithms Problems
- Data Structures & Algorithms I Actually Used Working at Tech Companies (2020) (Lobsters) (HN)
- Let’s implement a Bloom Filter (2020) (HN)
- Data Structures Part 1: Bulk Data (2019) (Lobsters)
- Data Structures Explained
- Introduction to Cache-Oblivious Data Structures (2018)
- The Daily Coding newsletter — Master JavaScript and Data Structures.
- Lectures Note for Data Structures and Algorithms (2019)
- Mechanically Deriving Binary Tree Iterators with Continuation Defunctionalization (2020)
- Segment Tree data structure
- Structure of a binary state tree (2020)
- Introductory data structures and algorithms
- Applying Textbook Data Structures for Real Life Wins (2020) (HN)
- Michael Scott — Nonblocking data structures lectures (2020) — Nonblocking concurrent data structures are an increasingly valuable tool for shared-memory parallel programming.
- Scal — High-performance multicore-scalable data structures and benchmarks. (Web)
- Hyperbolic embedding implementations
- Morphisms of Computational Constructs — Visual catalogue + story of morphisms displayed across computational structures.
- What is key-value store? (build-your-own-x) (2020)
- Lesser Known but Useful Data Structures
- Using Bloom filters to efficiently synchronize hash graphs (2020)
- Bloom Filters by Example (Code)
- Binary Decision Diagrams (HN)
- 3 Steps to Designing Better Data Structures (2020)
- Sparse Matrices (2019) (HN)
- Algorithms & Data Structures in C++
- Fancy Tree Traversals (2019)
- The Robson Tree Traversal (2019)
- Data structures and program structures
- cdb — Fast, reliable, simple package for creating and reading constant databases.
- PGM-index — Learned indexes that match B-tree performance with 83x less space. (HN) (Code)
- Structural and pure attributes
- Cache-Tries: O(1) Concurrent Lock-Free Hash Tries (2018)
By Bryan Guner on [August 24, 2021](https://medium.com/p/16ce4244be12).
Canonical link
Exported from [Medium](https://medium.com) on August 31, 2021.
