Python-VS-JS Cheat Sheet

Contents

• Versions

• Development Environments

• Running Programs

• Comments

• Semicolons

• Whitespace, Blocks

• Functions

• Arithmetic Operators

• Variables

• Data Types

• Arrays/Lists

• Slices

• Objects/Dicts

• String Formatting

• Booleans and Conditionals

• for Loops

• while Loops

• switch Statement

• if Conditionals

• Classes

Versions

JavaScript

The standard defining JavaScript (JS) is ECMAScript (ES). Modern browsers and NodeJS support ES6, which has a rich feature set. Older browsers might not support all ES6 features.

The website caniuse.com will show which browsers support specific JS features.

Python

Python 3.x is the current version, but there are a number of packages and sites running legacy Python 2.

On some systems, you might have to be explicit when you invoke Python about which version you want by running python2 or python3. The --version command line switch will tell you which version is running. Example:

$ python --version
Python 2.7.10
$ python2 --version
-bash: python2: command not found
$ python3 --version
Python 3.6.5

Using virtualenv or pipenv can really ease development painpoints surrounding the version. See Development Environments, below.

Development Environments

JavaScript

For managing project packages, the classic tool is npm. This is slowly being superseded by the newer yarn tool. Choose one for a project, and don't mix and match.

Python

For managing project packages and Python versions, the classic tool is virtualenv. This is slowly being superseded by the newer pipenv tool.

Running Programs

JavaScript

Running from the command line with NodeJS:

node program.js arg1 arg2 etc

In a web page, a script is referenced with a <script> HTML tag:

<script src="program.js"></script>

Python

Running from the command line:

python program.py arg1 arg2 etc

Comments

JavaScript

Single line:

// Anything after two forward slashes is a comment
print(2); // prints 2

Multi-line comments:

/* Anything between slash-star and
star-slash is a comment */

You may not nest multi-line comments.

Python

Single line:

# Anything after a # is a comment
print(2)  # prints 2

Python doesn't directly support multi-line comments, but you can effectively do them by using multi-line strings """:

"""
At this point we wish
to print out some numbers
and see where that gets us
"""
print(1)
print(2)

Semicolons

JavaScript

Javascript ends statements with semicolons, usually at the end of the line. I can also be effectively used to put multiple statements on the same line, but this is rare.

console.log("Hello, world!");

let x = 10;
console.log(x);

Javascript interpreters will let you get away without using semicolons at ends of lines, but you should use them.

Python

Python can separate statements by semicolons, though this is rare in practice.

print(1); print(2)   # prints 1, then 2

Whitespace, blocks

JavaScript

Whitespace has no special meaning. Blocks are declared with squirrely braces { and }.

if (x == 2) {
  console.log("x must be 2");
} else {
  if (x == 3) {
    console.log("x must be 3");
  } else {
    console.log("x must be something else");
  }
}

Python

Indentation level is how blocks are declared. The preferred method is to use spaces, but tabs can also be used.

if x == 2:
  print("x must be 2")
else:
  if x == 3:
    print("x must be 3")
  else:
    print("x must be something else")

Functions

JavaScript

Define functions as follows:

function foobar(x, y, z) {
  console.log(x, y, z);
  return 12;
}

An alternate syntax for functions is growing increasingly common, called arrow functions:

let hello = () => {
  console.log("hello");
  console.log("world");
};

hello(); // prints hello, then world

// Arrow functions with single parameters don't
// need parens around the parameter:
let printNum = (x) => {
  console.log(x);
};

// If you don't explicitly return a value, the value
// of the last expression in the function is used.
// Also, single-expression functions don't need
// braces around them.
let add = (x, y) => x + y;

console.log(add(4, 5)); // prints 9

Python

Define functions as follows:

def foobar(x, y, z):
  print(x, y, z)
  return 12

Python also supports the concept of lambda functions, which are simple functions that can do basic operations.

add = lambda x, y: x + y

print(add(4, 5)) # prints 9

Arithmetic Operators

JavaScript

Operator	Description
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo (remainder)
--	Pre-decrement, post-decrement
++	Pre-increment, post-increment
**	Exponentiation (power)
=	Assignment
+=	Addition assignment
-=	Subtraction assignment
*=	Multiplication assignment
/=	Division assignment
%=	Modulo assignment

Python

The pre- and post-increment and decrement are notably absent.

Operator	Description
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo (remainder)
**	Exponentiation (power)
=	Assignment
+=	Addition assignment
-=	Subtraction assignment
*=	Multiplication assignment
/=	Division assignment
%=	Modulo assignment

Variables

Javascript

Variables are created upon use, but should be created with the let or const keywords.

let x = 10;
const y = 30;

var is an outdated way of declaring variables in Javascript.

Python

Variables are created upon use.

x = 10

Data Types

JavaScript

let a = 12; // number
let b = 1.2; // number
let c = "hello"; // string
let d = "world"; // string
let e = true; // boolean
let f = null; // null value
let g = undefined; // undefined value

Multi-line strings:

let s = `this is a
multi-line string`;

Parameterized strings:

let x = 12;
console.log(`x is ${x}`); // prints "x is 12"

JS is weakly typed so it supports operations on multiple types of data at once.

"2" + 4; // string "24"
parseInt("2") + 4; // number 6
Number("2") + 4; // number 6

Python

a = 12       # int (integer)
b = 1.2      # float (floating point)
c = 'hello'  # str (string)
d = "world"  # str
e = False    # bool (boolean)
f = None     # null value

Multi-line strings:

s = """this is a
multi-line string"""

Parameterized strings:

x = 12
print(f'x is {x}')   # prints "x is 12"

Python is generally strongly typed so it it will often complain if you try to mix and match types. You can coerce a type with the int(), float(), str(), and bool() functions.

"2" + 4        # ERROR: can't mix types
int("2") + 4   # integer 6
"2" + str(4)   # string 24

Arrays/Lists

JavaScript

In JS, lists are called arrays.

Arrays are zero-based.

Creating lists:

let a1 = new Array(); // Empty array
let a2 = new Array(10); // Array of 10 elements
let a3 = []; // Empty array
let a4 = [10, 20, 30]; // Array of 3 elements
let a5 = [1, 2, "b"]; // No problem

Accessing:

console.log(a4[1]); // prints 20

a4[0] = 5; // change from 10 to 5
a4[20] = 99; // OK, makes a new element at index 20

Length/number of elements:

a4.length; // 3

Python

In Python, arrays are called lists.

Lists are zero-based.

Creating lists:

a1 = list()          # Empty list
a2 = list((88, 99))  # List of two elements
a3 = []              # Empty list
a4 = [10, 20, 30]    # List of 3 elements
a5 = [1, 2, "b"]     # No problem

Accessing:

print(a4[1])  # prints 20

a4[0] = 5;    # change from 10 to 5
a4[20] = 99;  # ERROR: assignment out of range

Length/Number of elements:

len(a4)   # 3

Slices

In Python, we can access parts of lists or strings using slices.

Creating slices:

a[start:end] # items start through end-1
a[start:]    # items start through the rest of the array
a[:end]      # items from the beginning through end-1
a[:]         # a copy of the whole array

Starting from the end: We can also use negative numbers when creating slices, which just means we start with the index at the end of the array, rather than the index at the beginning of the array.

a[-1]    # last item in the array
a[-2:]   # last two items in the array
a[:-2]   # everything except the last two items

Tuples

Python supports a read-only type of list called a tuple.

x = (1, 2, 3)
print(x[1])  # prints 2

y = (10,)    # A tuple of one element, comma required

List Comprehensions

Python supports building lists with list comprehensions. This is often useful for filtering lists.

a = [1, 2, 3, 4, 5]

# Make a list b that is the same as list a:
b = [i for i in a]  # Pretty boring

# Make a list c that contains only the
# even elements of a:
c = [i for i in a if i % 2 == 0]

Objects/Dicts

JavaScript

Objects hold data which can be found by a specific key called a property.

Creation:

let o1 = {}; // empty object
let o2 = { x: 12 }; // one property
let o3 = { y: "hello" }; // property quotes optional

let o4 = {
  // common multiline format
  a: 20,
  b: 1.2,
  foo: "hello",
};

Access:

console.log(o2.x); // prints 12
console.log(o4["foo"]); // prints hello

Python

Dicts hold information that can be accessed by a key.

Unlike objects in JS, a dict is its own beast, and is not the same as an object obtained by instantiating a Python class.

Creation:

o1 = {}           # empty dict
o2 = {"x": 12}    # one key
o3 = {y: "hello"} # ERROR: key quotes required,
                  # unless y is a variable
                  # that holds a value you wish
                  # to use as a key

o4 = {  # multiline format
  "a": 20,
  "b": 1.2,
  "foo": "hello"
}

Access:

print(o4["a"])  # Prints 20

Dot notation does not work with Python dicts.

String Formatting

JavaScript

Converting to different number bases:

let x = 237;
let x_binary = x.toString(2); // string '11101101'
let x_hex = x.toString(16); // string 'ed'

Controlling floating point precision:

let x = 3.1415926535;
let y = x.toFixed(2); // string '3.14'

Padding and justification:

let s = "Hello!";
let t = s.padStart(10, " "); // string '    Hello!'
let u = s.padEnd(10, " "); // string 'Hello!    '

let v = s.padStart(10, "*"); // string '****Hello!'

// Pad with leading zeroes
(12).toString(2).padStart(8, "0"); // string '00001100'

Parameterized strings:

let x = 3.1415926;
let y = "Hello";
let z = 67;

// 'x is 3.14, y is "Hello", z is 01000011'
let s = `x is ${x.toFixed(2)}, y is "${y}", z is ${z
  .toString(2)
  .padStart(8, "0")}`;

Python

Python has the printf operator % which is tremendously powerful. (If the operands to % are numbers, modulo is performed. If the left operand is a string, printf is performed.)

But even % is being superseded by the format function.

Tons of details at pyformat.info.

Also see printf-style String Formatting for a reference.

Booleans and Conditionals

JavaScript

Literal boolean values:

x = true;
y = false;

Boolean operators:

Operator	Definition
==	Equality
!=	Inequality
===	Strict equality
!==	Strict inequality
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

The concept of strict equality/inequality applies to items that might normally be converted into a compatible type. The strict tests will consider if the types themselves are the same.

0 == "0"; // true
0 === "0"; // false

0 == []; // true
0 === []; // false

0 == 0; // true
0 === 0; // true

Logical operators:

Operator	Description
!	Logical inverse, not
&&	Logical AND
`	`

The not operator ! can be used to test whether or not a value is "truthy".

!0; // true
!!0; // false
!1; // false
!null; // true
!"0"; // false, perhaps unexpectedly
!"x"; // false

Example:

if (a == 2 && b !== "") {
  // Something complicated
}

Python

Literal boolean values:

x = True
y = False

Boolean operators:

Operator	Definition
==	Equality
!=	Inequality
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

Logical operators:

Operator	Description
not	Logical inverse, not
and	Logical AND
or	Logical OR

The not operator can be used to test whether or not a value is "truthy".

not 0      # true
not not 0  # false
not 1      # false
not None;  # true
not "0"    # false, perhaps unexpectedly
not "x"    # false

Example:

if a == 2 and b != "":
  # Something complicated

for Loops

JavaScript

C-style for loops:

for (let i = 0; i < 10; i++) {
  console.log(i);
}

for-in loops iterate over the properties of an object or indexes of an array:

a = [10, 20, 30];

for (let i in a) {
  console.log(i);    # 0  1  2
  console.log(a[i]); # 10 20 30
}

b = {'x': 77, 'y': 88, 'z': 99};

for (let i in b) {
  console.log(i);    # x  y  z
  console.log(b[i]); # 77 88 99
}

for-of loops access the values within the array (as opposed to the indexes of the array):

a = [10, 20, 30];

for (let i of a) {
  console.log(i);  # 10 20 30
}

Python

for-in loops over an iteratable. This can be a list, object, or other type of iterable item.

Counting loops:

# Use the range() function to count:
for i in range(10):
  print(i)   # Prints 0-9

for i in range(20, 30):
  print(i)   # Prints 20-29

for i in range(-10, 20, 3):
  print(i)   # Print every 3rd number from -10 to 19

Iterating over other types:

# A list
a = [10, 20, 30]

# Print 10 20 30
for i in a:
  print(i)

# A dict
b = {'x':5, 'y':15, 'z':0}

# Print x y z (the keys of the dict)
for i in b:
  print(i)

while Loops

JavaScript

C-style while and do-while:

// Print 10 down to 0:

let x = 10;
while (x >= 0) {
  console.log(x);
  x--;
}

// Print 0 up to 9:
let x = 0;
do {
  console.log(x);
  x++;
while (x < 10);

Python

Python has a while loop:

# Print from 10 down through 0
x = 10
while x >= 0:
  print(x)
  x -= 1

switch Statement

JavaScript

JS can switch on various data types:

switch (x) {
  case "foo":
    console.log("x is foo, all right");
    break;
  case 23:
    console.log("but here x is 23");
    break;
  default:
    console.log("x is something else entirely");
}

Python

Python doesn't have a switch statement. You can use if-elif-else blocks.

A somewhat clumsy approximation of a switch can be constructed with a dict of functions.

def func1():
  print("case 1 is hit")

def func2():
  print("case 2 is hit")

def func3():
  print("case 3 is hit")

funcs = {
  "alpha": func1,
  "bravo": func2,
  "charlie": func3
};

x = "bravo"
funcs[x]()  # calls func2

if Conditionals

JavaScript

JS uses C-style if statements:

if (x == 10) {
  console.log("x is 10");
} else if (x == 20) {
  console.log("x is 20");
} else {
  console.log("x is something else");
}

Python

Python notably uses elif instead of else if.

if x == 10:
  print("x is 10")
elif x == 20:
  print("x is 20")
else:
  print("x is something else")

Classes

JavaScript

The current object is referred to by this.

Pre ES-2015, classes were created using functions. This is now outdated.

function Goat(color) {
  this.legs = 4;
  this.color = color;
}

g = new Goat("brown");

JS uses prototypal inheritance. Pre ES-2015, this was explicit, and is also outdated:

function Creature(type) {
  this.type = type;
}

// Make Goats inherit from Creature:
Goat.prototype = new Creature("mammal");

// Add a method:
Goat.prototype.jump = function () {
  console.log("I'm jumping! Yay!");
};

g = new Goat("red");
g.type; // "mammal", since Goat inherits from Creature
g.jump(); // "I'm jumping! Yay!"

Modern JS introduced the class keyword and a syntax more familiar to most other OOP languages. Note that the inheritance model is still prototypal inheritance; it's just that the details are hidden from the developer.

class Creature {
  constructor(type) {
    this.type = type;
  }
}

class Goat extends Creature {
  constructor(color) {
    super("mammal");
    this.legs = 4;
    this.color = color;
  }

  jump() {
    console.log("I'm jumping! Yay!");
  }
}

g = new Goat("orange");
g.type; // "mammal"
g.jump(); // "I'm jumping! Yay!"

JS does not support multiple inheritance since each object can only have one prototype object. You have to use a mix-in if you want to achieve similar functionality.

Python

The current object is referred to by self. Note that self is explicitly present as the first parameter in object methods.

Python 2 syntax:

class Creature:
  def __init__(self, type): # constructor
    self.type = type

class Goat(Creature):
  def __init__(self, color):
    # call super constructor
    Creature.__init__(self, "mammal")
    self.color = color

  def jump(self):
    print("I'm jumping! Yay!")

g = Goat("green")
g.type    # mammal
g.jump()  # I'm jumping! Yay!

Python 3 syntax includes the new super() keyword to make life easier.

class Creature:
  def __init__(self, type): # constructor
    self.type = type

class Goat(Creature):
  def __init__(self, color):
    # call super constructor
    super().__init__("mammal")  # <-- Nicer!
    self.color = color

  def jump(self):
    print("I'm jumping! Yay!")

g = Goat("green")
g.type    # mammal
g.jump()  # I'm jumping! Yay!

Python supports multiple inheritance.