Basic Types

This guide is in the early stages, feedback welcomed on Githubopen in new window


Let's start by running code through the Python interpreter from Mojo to get a PythonObjectopen in new window back:

from python import Python

x = Python.evaluate('5 + 10')

x is represented in memory the same way as if we ran this in Python:

x = 5 + 10

In Python, x is actually a pointer to heap allocated memory.

CS Fundamentals

stack and heap memory are really important concepts to understand, this YouTube videoopen in new window does a fantastic job of explaining it visually.

If the video doesn't make sense, for now you can use the mental model that:

  • stack memory is very fast but small, the size of the values are static and can't change at runtime
  • pointer is an address to lookup the value somewhere else in memory
  • heap memory is huge and the size can change at runtime, but needs a pointer to access the data which is relatively slow

These concepts will make more sense over time

You can access all the Python keywords by importing builtins:

py = Python.import_module("builtins")

py.print("this uses the python print keyword")
this uses the python print keyword

We can now use the type builtin from Python to see what the dynamic type of x is:

<class 'int'>

We can read the address that is stored in x on the stack using the Python builtin id


This is pointing to a C object in Python, and Mojo behaves the same when using a PythonObject, accessing the value actually uses the address to lookup the data on the heap which comes with a performance cost.

This is a simplified representation of how the C Object being pointed to would look if it were a Python dict:

# md-notebook:skip
heap = {
    44601345678945: {
        "type": "int",
        "ref_count": 1,
        "size": 1,
        "digit": 8,

On the stack the simplified representation of x would look like this:

# md-notebook:skip
    { "frame": "main", "variables": { "x": 44601345678945 } }

x contains an address that is pointing to the heap object

In Python we can change the type dynamically:

x = "mojo"

The object in C will change its representation:

# md-notebook:skip
heap = {
    44601345678945 : {
        "type": "string",
        "ref_count": 1,
        "size": 4,
        "ascii": True,
        # utf-8 / ascii for "mojo"
        "value": [109, 111, 106, 111]
        # ...

Mojo also allows us to do this when the type is a PythonObject, it works the exact same way as it would in a Python program.

This allows the runtime to do nice convenient things for us

  • once the ref_count goes to zero it will be de-allocated from the heap during garbage collection, so the OS can use that memory for something else
  • an integer can grow beyond 64 bits by increasing size
  • we can dynamically change the type
  • the data can be large or small, we don't have to think about when we should allocate to the heap

However this also comes with a penalty, there is a lot of extra memory being used for the extra fields, and it takes CPU instructions to allocate the data, retrieve it, garbage collect etc.

In Mojo we can remove all that overhead:

Mojo 🔥

x = 5 + 10

We've just unlocked our first Mojo optimization! Instead of looking up an object on the heap via an address, x is now just a value on the stack with 64 bits that can be passed through registers.

This has numerous performance implications:

  • All the expensive allocation, garbage collection, and indirection is no longer required
  • The compiler can do huge optimizations when it knows what the numeric type is
  • The value can be passed straight into registers for mathematical operations
  • There is no overhead associated with compiling to bytecode and running through an interpreter
  • The data can now be packed into a vector for huge performance gains

That last one is very important in today's world, let's see how Mojo gives us the power to take advantage of modern hardware.


SIMD stands for Single Instruction, Multiple Data, hardware now contains special registers that allow you do the same operation across a vector in a single instruction, greatly improving performance, let's take a look:

y = SIMD[DType.uint8, 4](1, 2, 3, 4)
[1, 2, 3, 4]

In the definition [DType.uint8, 4] are known as parameters which means they must be compile-time known, while (1, 2, 3, 4) are the arguments which can be compile-time or runtime known.

For example user input or data retrieved from an API is runtime known, and so can't be used as a parameter during the compilation process.

In other languages argument and parameter often mean the same thing, in Mojo it's a very important distinction.

This is now a vector of 8 bit numbers that are packed into 32 bits, we can perform a single instruction across all of it instead of 4 separate instructions:

y *= 10
[10, 20, 30, 40]

CS Fundamentals

Binary is how your computer stores memory, with each bit representing a 0 or 1. Memory is typically byte addressable, meaning that each unique memory address points to one byte, which consists of 8 bits.

This is how the first 4 digits in a uint8 are represented in hardware:

  • 1 = 00000001
  • 2 = 00000010
  • 3 = 00000011
  • 4 = 00000100

Binary 1 and 0 represents ON or OFF indicating an electrical charge in the tiny circuits of your computer.

Check this videoopen in new window if you want more information on binary.

We're packing the data together with SIMD on the stack so it can be passed into a SIMD register like this:

00000001 00000010 00000011 00000100

The SIMD registers in modern CPU's are huge, lets see how big it is on this computer:

from import simdbitwidth

That means we could pack 32 x 8bit numbers together and perform a calculation on all of it with a single instruction.

You can also initialize SIMD with a single argument:

z = SIMD[DType.uint8, 4](1)
[1, 1, 1, 1]


Scalar just means a single value, you'll notice in Mojo all the numerics are SIMD scalars:

# md-notebook:skip
var x = UInt8(1)
x = "will cause an error"
/tmp/md-notebook/main.mojo:48:5: error: invalid redefinition of 'x'
    var x = UInt8(1)
/tmp/md-notebook/main.mojo:11:5: note: previous definition here
    x = Python.evaluate('5 + 10')

UInt8 is just an alias for SIMD[DType.uint8, 1], you can see all the numeric SIMD types imported by default hereopen in new window:

  • Int8
  • Int16
  • Int32
  • Int64
  • UInt8
  • UInt16
  • UInt32
  • UInt64
  • Float16
  • BFloat16
  • Float32
  • Float64

Also notice when we try and change the type it throws an error, this is because Mojo is strongly typed

If we use existing Python modules, it will give us back a PythonObject that behaves the same loosely typed way as it does in Python:

np = Python.import_module("numpy")

arr = np.ndarray([5])
arr = "this will work fine"
[4.90821123e-310 0.00000000e+000 5.61870786e-310 3.37446836e-321
this will work fine


In Mojo String is heap allocated:

s = String("Mojo🔥")

String is contains a pointer to heap allocated data, this means we can load a huge amount of data into it limited only by your free RAM, and change the size of the data dynamically during runtime.

Let's cause a type error so you can see the data type underlying the String:

# md-notebook:skip
var x = s._buffer
x = 20
/tmp/md-notebook/main.mojo:61:10: error: cannot implicitly convert 'List[SIMD[si8, 1]]' value to 'PythonObject' in assignment
    x = s._buffer

List is similar to a Python List, here it's storing multiple int8's that represent the characters, let's print the first character:


Now lets take a look at the decimal representation:

from builtin.string import ord

decimal = ord(s[0])

That's the ASCII code shown in this tableopen in new window

We can build our own string this way, we can put in 78 which is N and 79 which is O

var word = List[Int8]()

word.append(0) # Must null terminate the String

We can use a String to copy the data to another location in memory, and it can now use the data as a String:

var word_str = String(word)

Because it points to a different location in heap memory, changing the original vector won't change the value retrieved by the reference:

word[1] = 78

There is also a StringLiteral type, it's written directly into the binary, when the program starts it's loaded into read-only memory, which means it's constant and lives for the duration of the program:

var literal = "This is my StringLiteral"
This is my StringLiteral

This is my StringLiteral

Force an error to see the type:

# md-notebook:skip
literal = 20
/tmp/md-notebook/main.mojo:92:15: error: cannot implicitly convert 'IntLiteral' value to 'StringLiteral' in assignment
    literal = 20

One thing to be aware of is that an emoji is actually four bytes, so we need a slice of 4 to have it print correctly:

emoji = String("🔥😀")
print("fire:", emoji[0:4])
print("smiley:", emoji[4:8])
fire: 🔥
smiley: 😀

fire: 🔥 smiley: 😀

Check out Maxim Zaks Blog postopen in new window for more details.

Other Builtins

These are all of the other builtin types not discussed which are accessible without importing anything, the type can be inferred, but are explicit here for demonstration, for example let bool: Bool = True can just be let bool = True:


Standard Bool type

var bool: Bool = True
print(bool == False)


Int is the same size as your architecture e.g. on a 64 bit machine it's 64 bits

var i: Int = 2 

It can also be used as an index:

var list2 = List[Int]()



Float literals can now be any size and are not limited to your CPU bit size:

var float: FloatLiteral = 33244677779764631144466467797946.334646469
var f32 = Float32(float)


When you initialize the list the types can be inferred, however when retrieving an item you need to provide the type as a parameter:

var list = [1, "Mojo🔥"]
var item1 = list.get[1, StringLiteral]()



var tup = (1, "Mojo", 3)
var item2 = tup.get[2, Int]()


A slice follows the python convention of:


So for example using Python syntax:

var original = String("MojoDojo")

You can also represent as:

var slice_expression = slice(0, 4)


And to get every second letter:



var slice_expression2 = slice(0, 4, 2)


The error type is very simplistic, we'll go into more details on errors in a later chapter:

# md-notebook:skip
def return_error():
    raise Error("This returns an Error type")

Unhandled exception caught during execution: This returns an Error type


  1. Use the Python interpreter to calculate 2 to the power of 8 in a PythonObject and print it
  2. Using the Python math module, return pi to Mojo and print it
  3. Initialize two single floats with 64 bits of data and the value 2.0, using the full SIMD version, and the shortened alias version, then multiply them together and print the result.
  4. Create a loop using SIMD that prints four rows of data that looks like this:

In Mojo you can create a loop like this:

for i in range(4):


Exercise 1

var pow = Python.evaluate("2 ** 8") 

Exercise 2

var math = Python.import_module("math")

var pi = math.pi

Exercise 3

var left = Float64(2.0)
var right = SIMD[DType.float64, 1](2.0)

var res = left * right

Exercise 4

for i in range(4):
    simd = SIMD[DType.uint8, 4](0)
    simd[i] = 1
[1, 0, 0, 0]
[0, 1, 0, 0]
[0, 0, 1, 0]
[0, 0, 0, 1]