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Deep Learning From Scratch - Theory and Implementation


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Deep Learning From Scratch: Theory and Implementation

In this tutorial, we develop the mathematical and algorithmic underpinnings of deep neural networks from scratch and implement our own neural network library in Python, mimicing the TensorFlow API. I do not assume that you have any preknowledge about machine learning or neural networks. However, you should have some preknowledge of calculus, linear algebra, fundamental algorithms and probability theory on an undergraduate level. If you get stuck at some point, please leave a comment.

By the end of this text, you will have a deep understanding of the math behind neural networks and how deep learning libraries work under the hood.

I have tried to keep the code as simple and concise as possible, favoring conceptual clarity over efficiency. Since our API mimics the TensorFlow API, you will know how to use TensorFlow once you have finished this text, and you will know how TensorFlow works under the hood conceptually (without all the overhead that comes with an omnipotent, maximally efficient machine learning API).

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Computational Graphs

We shall start by defining the concept of a computational graph, since neural networks are a special form thereof. A computational graph is a directed graph where the nodes correspond to operations or variables. Variables can feed their value into operations, and operations can feed their output into other operations. This way, every node in the graph defines a function of the variables.

The values that are fed into the nodes and come out of the nodes are called tensors, which is just a fancy word for a multi-dimensional array. Hence, it subsumes scalars, vectors and matrices as well as tensors of a higher rank.

Let's look at an example. The following computational graph computes the sum z of two inputs x and y. Here, x and y are input nodes to z and z is a consumer of x and y. z therefore defines a function z:R2R where z(x,y)=x+y.


The concept of a computational graph becomes more useful once the computations become more complex. For example, the following computational graph defines an affine transformation z(A,x,b)=Ax+b.



Every operation is characterized by three things:

  • A compute function that computes the operation's output given values for the operation's inputs
  • A list of input_nodes which can be variables or other operations
  • A list of consumers that use the operation's output as their input

Let's put this into code:

class Operation:
"""Represents a graph node that performs a computation.
An `Operation` is a node in a `Graph` that takes zero or
more objects as input, and produces zero or more objects
as output.
def __init__(self, input_nodes=[]):
"""Construct Operation
self.input_nodes = input_nodes
# Initialize list of consumers (i.e. nodes that receive this operation's output as input)
self.consumers = []
# Append this operation to the list of consumers of all input nodes
for input_node in input_nodes:
# Append this operation to the list of operations in the currently active default graph
def compute(self):
"""Computes the output of this operation.
"" Must be implemented by the particular operation.

Some elementary operations

Let's implement some elementary operations in order to become familiar with the Operation class (and because we will need them later).

class add(Operation):
"""Returns x + y element-wise.
def __init__(self, x, y):
"""Construct add
x: First summand node
y: Second summand node
super().__init__([x, y])
def compute(self, x_value, y_value):
"""Compute the output of the add operation
x_value: First summand value
y_value: Second summand value
return x_value + y_value
Matrix multiplication
class matmul(Operation):
"""Multiplies matrix a by matrix b, producing a * b.
def __init__(self, a, b):
"""Construct matmul
a: First matrix
b: Second matrix
super().__init__([a, b])
def compute(self, a_value, b_value):
"""Compute the output of the matmul operation
a_value: First matrix value
b_value: Second matrix value

In both of these operations, we assume that the tensors are NumPy arrays, in which the element-wise addition and matrix multiplication (.dot) are already implemented for us.


Not all the nodes in a computational graph are operations. For example, in the affine transformation graph, A, x and b are not operations. Rather, they are inputs to the graph that have to be supplied with a value once we want to compute the output of the graph. To provide such values, we introduce placeholders.

class placeholder:
"""Represents a placeholder node that has to be provided with a value
when computing the output of a computational graph
def __init__(self):
"""Construct placeholder
self.consumers = []
# Append this placeholder to the list of placeholders in the currently active default graph


In the affine transformation graph, there is a qualitative difference between x on the one hand and A and b on the other hand. While x is an input to the operation, A and b are parameters of the operation, i.e. they are intrinsic to the graph. We will refer to such parameters as Variables.

class Variable:
"""Represents a variable (i.e. an intrinsic, changeable parameter of a computational graph).
def __init__(self, initial_value=None):
"""Construct Variable
initial_value: The initial value of this variable
self.value = initial_value
self.consumers = []
# Append this variable to the list of variables in the currently active default graph

The Graph class

Finally, we'll need a class that bundles all the operations, placeholders and variables together. When creating a new graph, we can call its as_default method to set the _default_graph to this graph. This way, we can create operations, placeholders and variables without having to pass in a reference to the graph everytime.

class Graph:
"""Represents a computational graph
def __init__(self):
"""Construct Graph"""
self.operations = []
self.placeholders = []
self.variables = []
def as_default(self):
global _default_graph
_default_graph = self


Let's now use the classes we have built to create a computational graph for the following affine transformation:

# Create a new graph
# Create variables
A = Variable([[1, 0], [0, -1]])
b = Variable([1, 1])
# Create placeholder
x = placeholder()
# Create hidden node y
y = matmul(A, x)
# Create output node z
z = add(y, b)

Computing the output of an operation

Now that we are confident creating computational graphs, we can start to think about how to compute the output of an operation.

Let's create a Session class that encapsulates an execution of an operation. We would like to be able to create a session instance and call a run method on this instance, passing the operation that we want to compute and a dictionary containing values for the placeholders:

session = Session()
output =, {
    x: [1, 2]

This should compute the following value:


In order to compute the function represented by an operation, we need to apply the computations in the right order. For example, we cannot compute z before we have computed y as an intermediate result. Therefore, we have to make sure that the operations are carried out in the right order, such that the values of every node that is an input to an operation o has been computed before o is computed. This can be achieved via post-order traversal.

import numpy as np
class Session:
"""Represents a particular execution of a computational graph.
def run(self, operation, feed_dict={}):
"""Computes the output of an operation
operation: The operation whose output we'd like to compute.
feed_dict: A dictionary that maps placeholders to values for this session
# Perform a post-order traversal of the graph to bring the nodes into the right order
nodes_postorder = traverse_postorder(operation)
# Iterate all nodes to determine their value
for node in nodes_postorder:
if type(node) == placeholder:
# Set the node value to the placeholder value from feed_dict
node.output = feed_dict[node]
elif type(node) == Variable:
# Set the node value to the variable's value attribute
node.output = node.value
else: # Operation
# Get the input values for this operation from the output values of the input nodes
node.inputs = [input_node.output for input_node in node.input_nodes]
# Compute the output of this operation
node.output = node.compute(*node.inputs)
# Convert lists to numpy arrays
if type(node.output) == list:
node.output = np.array(node.output)
# Return the requested node value
return operation.output
def traverse_postorder(operation):
"""Performs a post-order traversal, returning a list of nodes
in the order in which they have to be computed
operation: The operation to start traversal at
nodes_postorder = []
def recurse(node):

Let's test our class on the example from above:

session = Session()
output =, {
x: [1, 2]
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