This course is an introduction to computer science and programming in Python. Upon successful completion of this course, you will be able to: 1. Take a new computational problem and develop a plan to solve it through problem understanding and decomposition. 2. Follow a design creation process that includes specifications, algorithms, and testing. 3. Code, test, and debug a program in Python, based on your design. Important computer science concepts such as problem solving (computational thinking), problem decomposition, algorithms, abstraction, and software quality are emphasized throughout. The Python programming language and video games are used to demonstrate computer science concepts in a concrete and fun manner. However, a learner can take the knowledge and skills from this course and apply them to non-game problems, other programming languages, and other computer science courses. You do not need any previous programming, Python, or video game experience. However, some computer skills (e.g., mouse, keyboard, document editing), knowledge of algebra, attention to detail (as with many technical subjects), and a “just give it a try” spirit will be keys to your success. Despite the use of video games for all the programming examples, PVG is not about computer games. PVG will still provide valuable knowledge and skills for non-game computational problems. The interactive learning objects (ILO) of the course provide automatic, context-specific guidance and feedback, like a virtual teaching assistant, as you develop problem descriptions, algorithms, and functional test plans. The course forums will be supported by the creators of the course, to help you succeed. All videos, assessments, and ILOs are available free of charge. There is an optional certificate available for a fee.
Python Assignment Statement
Loading...
From the course by University of Alberta
Problem Solving, Programming, and Video Games
6 ratings
From the lesson
Module 3: Hacking Version 2
In Module 3, you will identify solution issues in your game. You will apply a second form of the abstraction problem-solving technique, called using templates, to solve a solution issue by using a graphics library. You will then use lexics, syntax, and semantics to learn two new Python statements (assignment, import), two new Python expressions (binary expression, attribute reference), and one new Python type (module). You will employ these Python constructs and a simple graphics library to write, test, and debug Hacking Version 2.
Meet the Instructors
Duane SzafronProfessor
Computing Science
Paul LuProfessor
Computing Science
In this video, we introduce one new statement, "The assignment statement."
An assignment statement is used to evaluate
an expression and bind an identifier to the result object.
Consider this program.
This program prompts the user to enter a color,
creates a string object that represents the color,
displays the color, and then indicates that this is the user's pick.
Purple is my favorite color.
I want to display the color string later in
the program rather than right after it was input.
I want to input two colors before I display either of them.
The program output should look like this.
If we could bind an identifier to each color string returned by the input function call,
we could use these identifiers anytime later in
the program to refer to these string objects.
The code would look like this.
This code uses a new statement called an "assignment
statement" to bind the identifier favorite to the first color.
It uses a second assignment statement to bind
the identifier second favorite to the second color.
Since we have two kinds of statements,
expression statement and assignment statement,
I will put them in a syntax diagram.
Now, I will show you the syntax and semantics of an assignment statement.
We will only discuss the simplest form of assignment statement
which binds a single target to an evaluated expression object.
Here is the syntax diagram.
An assignment statement consists of a target,
an assignment symbol, and an expression.
The simplest form of target is an identifier.
We will discuss other kinds of targets in a future lesson.
The assignment symbol is a delimiter token consisting of a single equal sign.
It does not designate equality in Python,
and is usually pronounced assigned or gets rather than equals.
The semantics of an assignment statement whose target is an identifier are: one,
evaluate the expression to obtain a result object, two,
if the identifier is not in the namespace,
add it to the namespace,
three, bind the identifier to this result object.
We will discuss how to bind targets that are not identifiers in a future lesson.
I will show you how the Python interpreter interprets this program.
The Python interpreter first uses
lexical analysis to successfully create a sequence of tokens.
It then apply syntax analysis to combine these tokens into statements.
Statement one is an assignment statement whose target is the identifier favorite,
followed by the assignment delimiter,
and it ends with an expression,
which is a function call to the builtin function input.
Statement two is also an assignment statement.
The other four statements are expression statements.
Since the program is syntactically valid,
the interpreter applies semantic analysis to
evaluate the program one statement at a time.
Here is a diagram of the namespace when the program starts,
containing pre-bound identifiers, input, and print.
To evaluate statement one,
the interpreter applies the semantics for an assignment statement.
First, it evaluates the expression.
To evaluate the expression,
the interpreter uses function calls semantics.
Let's skip the details of the function call semantics.
The result is that the input function code displays its prompt string and
returns a string that represents whatever keys
the user typed before pressing the enter key.
In this case, the string is purple.
The interpreter then continues with step two of
the assignment statement semantics by looking for
the identifier favorite in the namespace.
Since the identifier favorite is not in the namespace,
the interpreter adds it.
The interpreter then apply step three of
the semantic rule to bind favorite to the string object purple.
Statement two is evaluated in a similar way,
so second favorite is bound to the return object
of the second input function call, the string red.
Statement three is an expression statement that calls
the builtin function print to display the literal string, You picked.
Statement four is also an expression statement.
When the interpreter applies the function call semantics,
it evaluates the function argument,
which is an identifier.
Dereferencing the identifier favorite is
exactly the same as dereferencing the print identifier.
Recall the semantics for evaluating an identifier expression.
Since the identifier favorite is in the namespace,
it is dereferenced to obtain the object it is bound to,
the string object "purple."
Since this object is the argument to a print function call, purple is displayed.
Statement five is an expression statement that calls
the builtin function print to display another literal string "and."
Statements six is similar to statement four,
except the interpreter dereferences
the identifier second favorite to obtain the red string and displays it.
An identifier that is bound can be rebound to a different object.
If the semantic rule for binding an identifier is
applied to an identifier that is already bound,
then the identifier is already in the namespace,
so it is not added.
However, it is rebound to the object that the expression evaluated to.
The previous binding is gone.
Consider this program.
When this program starts,
the namespace contains three predefined identifier's;
len, id, and print,
each bound to its own function object.
This program has no lexical or syntax errors.
Statement one is an assignment statement.
When statement one is evaluated,
the expression consisting of the identifier id is evaluated.
The id identifier is dereferenced to obtain the id function object.
The target identifier len is in the namespace,
so it is not added.
Len is then bound to the id function object.
In statement two, the expression consists of a function
called print applied to an argument expression,
which is a function called len with argument expression the literal string, "hello."
The literal string evaluates to the string object 'hello'.
The identifier len is then dereferenced in the namespace to obtain the id function,
this id function is applied to the hello
string object to obtain the hard to remember identity object.
Notice that the length function is not applied since
the len identifier was rebound to the id function object.
This id function returns the hard to remember identity object of the 'hello' string.
The print function call displays this identity object in human readable form.
I did this to show you that any identifier can be bound to any object.
However, don't try this at home.
You should never rebind an identifier that is pre-bound before our code is evaluated.
You can use the builtin dir function to
examine the namespace as you evaluate Python expressions.
Remember, typing dir of underscore, underscore, builtins,
underscore displays the names from the namespace that are pre abound to builtin objects.
If I call dir in a shell with no argument,
it displays some names that start with two underscores and end with two underscores.
The names that appear depend on your Python implementation.
If I bind an identifier and rebind len,
and then call the dir function again,
it now includes favorite to indicate that it has been bound.
It also includes len to indicate that len has been
rebound to a different object then it's pre binding object.
We won't investigate the other names that contain underscores in this course.
In this video, I have described the syntax and semantics of the assignment statement.
An assignment statement can be used to bind an identifier to any object,
so that object can be used later in the program.
Explore our Catalog
Join for free and get personalized recommendations, updates and offers.
Coursera provides universal access to the world’s best education,
partnering with top universities and organizations to offer courses online.
© 2019 Coursera Inc. All rights reserved.
