02 - Expressions A - Python for Everybody Course

Summary

This video script delves into the fundamental components of Python programming, including constants, reserved words, and variables. It explains how constants are unchanging values used for specific calculations, while variables are memory allocations with names that can store and change values. The script emphasizes the importance of understanding assignment statements and choosing meaningful variable names to enhance code readability. It also clarifies that Python does not interpret the meaning behind variable names, highlighting the need for clear and descriptive naming conventions to aid programmers in understanding the code's purpose.

Takeaways

• 📌 Constants in Python are unchanging values like numbers or strings that are used as starting points for calculations or conditions.
• 🔑 Reserved words in Python have special meanings and are used for specific programming constructs, such as 'if' for conditional execution.
• 💾 Variables are used to allocate memory and store data. They are named and can hold different values over time, hence the term 'variable'.
• 👉 The assignment statement in Python is like an arrow, indicating where to store the value, and it evaluates the right-hand side before assigning it to the left-hand side.
• 📝 Variable names in Python can start with a letter or underscore, and can contain letters, numbers, and underscores (except as the first character). They are case-sensitive.
• 🚫 Avoid using underscores as the first character of a variable name, as they are often reserved for Python's own variables.
• 🔑 Choosing descriptive variable names can improve code readability, but remember that Python does not understand the meaning behind the names.
• 🔄 Variables can be reassigned new values, which overwrites the old values, demonstrating the 'variable' nature of variables.
• 🔢 Constants, reserved words, and variables are fundamental building blocks of Python programming.
• 📝 The script emphasizes the importance of understanding assignment statements and the role of variables in Python programming.
• 🤔 The speaker warns against attributing too much intelligence to Python based on mnemonic variable names, as Python is indifferent to the meaning of the names.

Q & A

What are constants in Python and why are they important?

Constants in Python are values that do not change, such as numbers or strings. They are important because they provide a way to start calculations and can be used to set conditions, like checking if the number of hours worked is greater than 40.

What is the role of reserved words in Python programming?

Reserved words in Python have special meanings and are used by Python to implement conditional execution and other language features. They are essential for the language to function properly.

How does Python allocate memory for variables?

Python allocates memory for variables through the assignment statement. It assigns a value to a variable by finding a piece of memory, labeling it with the variable name, and then storing the value in that memory.

What is the significance of the assignment statement in Python?

The assignment statement is significant because it assigns a value to a variable. It is important to think of it as having an arrow, indicating the direction of the assignment from the value to the variable.

Can variables in Python hold more than one value?

Yes, in Python, variables can hold more than one value, especially when using collections like lists or dictionaries, which will be covered in later chapters.

What are the rules for naming variables in Python?

Variable names in Python can start with a letter or an underscore, but not with a number or special character. They can contain letters, numbers, and underscores after the first character. Variable names are case-sensitive, and using meaningful names is recommended for better readability.

Why is it not recommended to start variable names with an underscore?

Starting variable names with an underscore is not recommended because it is often reserved for variables that communicate with Python itself. Using underscores for normal variables can lead to confusion and is not a common practice.

What is the difference between using short, meaningless variable names and using meaningful or mnemonic variable names?

Short, meaningless variable names might be convenient and easy to type but can make the code less readable and harder to understand. Meaningful or mnemonic variable names improve code readability and help others understand the purpose of the variable.

How does Python handle the evaluation of expressions in an assignment statement?

Python evaluates the entire right-hand side of an assignment statement before changing the left-hand side. This means it calculates the expression first and then assigns the result to the variable on the left.

Why is it important to understand the 'arrow nature' of the assignment statement in Python?

Understanding the 'arrow nature' of the assignment statement is important because it clarifies the direction of value assignment and helps prevent confusion when the same variable appears on both sides of the statement.

What is the purpose of the print function in Python?

The print function in Python is used to output data to the console. It is a built-in function that takes the data to be printed as an argument within parentheses.

Outlines

00:00

📚 Introduction to Python Fundamentals

This paragraph introduces the basic concepts of Python programming, including constants, which are unchanging values like numbers and strings used in calculations. It explains the importance of constants in programming and demonstrates their usage with the print function. The paragraph also covers reserved words, which have special meanings in Python and are crucial for conditional execution. Variables are introduced as a way to allocate memory and store data, with an emphasis on the assignment statement's role in assigning values to variables. The rules for creating variable names are outlined, highlighting the importance of choosing meaningful and descriptive names to enhance code readability.

05:02

🔍 Deep Dive into Variables and Assignments

The second paragraph delves deeper into the concept of variables and assignment statements. It discusses how variables can hold values that can be changed, illustrating this with examples of reassigning values to variables. The paragraph emphasizes the 'arrow' nature of assignment statements, explaining that the right-hand side of the statement is evaluated before the left-hand side is updated. It also touches on the potential for errors in variable naming and usage, and the importance of clear, mnemonic variable names to aid understanding. The summary concludes with an example of how meaningful variable names can greatly improve the readability and maintainability of code.

Keywords

💡Constants

Constants in programming are values that do not change throughout the execution of a program. They are used to set fixed values such as numbers or strings that can be referenced in various parts of the code. In the script, constants are mentioned as integral to starting calculations, with examples given like '40' in a condition to check if work hours exceed a threshold, and 'hello world' as a string constant.

💡Reserved Words

Reserved words in programming are terms that have special meaning to the language and are used for its syntax. They cannot be used as variable names. The script explains that in Python, reserved words are crucial for the language to understand and execute conditional statements, emphasizing their importance in the foundational structure of Python programming.

💡Variables

Variables are fundamental to programming as they allow the programmer to store data that can change over time. The script describes variables as a way to allocate memory and assign a name to it, which can then be used to store values. Variables are manipulated through assignment statements, which are likened to arrows indicating the direction of value assignment.

💡Assignment Statement

An assignment statement is used to assign a value to a variable. The script illustrates this concept by explaining that the assignment has an 'arrow' nature, indicating the direction of value assignment. For example, 'X = 12.2' is not saying that X is forever 12.2, but rather it is placing the value 12.2 into a memory location labeled by the variable name X.

💡Syntax

Syntax refers to the set of rules that defines how to structure statements written in a programming language. The script mentions syntax in the context of constants, explaining how the print function in Python uses syntax to display output, such as 'print(123)' which would display the number 123.

💡Conditional Execution

Conditional execution is a programming concept where the flow of a program is altered based on certain conditions. The script mentions that reserved words in Python are used to implement conditional execution, which is a key part of programming logic to perform different actions based on variable values or expressions.

💡Mnemonic Variable Names

Mnemonic variable names are those that are chosen to be meaningful and descriptive of the data they hold, making the code easier to understand. The script emphasizes the importance of using mnemonic names to help the reader of the code understand what each variable represents, such as using 'payroll' or 'hours' instead of single-letter variable names.

💡Function

A function in programming is a reusable block of code that performs a specific task. The script refers to 'print' as a built-in function in Python that outputs data to the console. It is used to display the results of expressions or variables, enhancing the readability and debugging process of the code.

💡Expression

An expression in programming is a combination of values, variables, operators, and calls to functions that can be evaluated to a single value. The script discusses expressions in the context of assignment statements, where the right-hand side of the assignment is evaluated to determine the new value for the variable on the left-hand side.

💡Variable Naming Conventions

Variable naming conventions are the rules and best practices for choosing variable names in programming. The script outlines several rules, such as starting with a letter or underscore, using letters, numbers, and underscores thereafter, and avoiding the use of case sensitivity as the only differentiator. It also advises against using underscores as the first character unless necessary for built-in variables.

Highlights

Introduction to the building blocks of Python: variables, constants, statements, and expressions.

Constants are unchanging elements such as numbers or strings used in calculations.

Constants like 40 hours can trigger certain actions in programming logic.

Demonstration of the print function to output constants.

Explanation of reserved words in Python and their special meanings for the language.

Variables as a way to allocate memory and store data with a name.

Assignment statements are used to assign values to variables, visualized with an arrow indicating direction.

Variables can hold multiple values, as seen in later chapters on collections.

Rules for creating variable names, starting with a letter or underscore, and following with letters, numbers, or underscores.

Case-sensitivity in variable names and the convention against using case as the only differentiator.

The importance of choosing descriptive variable names for clarity in programming.

The misconception that Python understands the meaning of variable names like 'payroll'.

Examples of code with assignment statements, demonstrating how values are updated and printed.

The process of how assignment statements evaluate the right-hand side before updating the left-hand side.

The impact of using mnemonic variable names to enhance code readability and understanding.

A cautionary note about not attributing intelligence to Python based on variable names.

The concept of complex expressions and their role in Python programming.

Music indicating a transition or conclusion in the lecture.

Transcripts

hello and welcome to Chapter two now

we're going to continue to talk about

the building blocks of Python variables

constants statements expressions etc the

first thing we have to talk about is

constants these are just things we call

constants because they don't change

their numbers strings etc and we use

them to sort of start calculations or

you know if if something is greater than

40 hours we're going to do something and

so 40 is the constant in that situation

so we have 123 we have 98.6 we have

hello world which is a string by

enclosing it in quotes we pass each of

these things to the print function and a

side of the respect to the print

function is that we see the output

so print hundred 23 prints out 123 print

98.6 prints it out so these are just

really the syntax of constants and

without constants we can't write really

much of anything the other sort of

foundational notion of any programming

language are the reserved words and like

I said before reserved words are these

special words where Python is listening

for them and there are as very special

meanings so in Python seas if it's not

just any other word it means how Python

implements conditional execution

variables are the third building block

and that is a a way that you can ask

Python to allocate a piece of memory and

then give it a name and you can put

stuff in there what you sometimes you

just put one value later we'll see when

we do a collections in chapters 8 9 we

will see that more than one value can be

put into a variable and the variable the

consider how we control the variable is

through the assignment statement and as

I said before it's important to think of

the assignment statement as having an

arrow to it so this is not saying X for

all time is the same as 12.2 what it's

saying is take 12 point to find a place

find some memory in your computer there

mister Python give it a label X we get

to choose the X that's the variable part

we chose it right and then stick 12 in

it and then the same is true for 14

we'll find in there another spot name it

Y and then put a 14 in there so think of

this as an arrow every time you see that

equality

the assignment in an assignment

statement now these variables hold one

value so now if we have these three

statements these two and then the third

one executes it says put 100 into X but

that wipes out the old value of twelve

point two and it rewrites it with a hunt

with a hundred and so we can change the

variables that's another reason that we

call them variable there are some names

now some rules for making variable names

you can start with a letter or an

underscore we tend not to as normal

programmers use underscore we tend to

reserve those four variables that we use

to communicate with Python itself so

we're making up a variable we tend not

to use underscores as a pre first

character you can have letters and

numbers and underscores after the first

character and their case-sensitive but

it's really a bad idea to use cases the

only differentiator so in this case spam

eggs spam 23 and underscores meter alt

or legit we would probably not use this

one unless we were actually doing it

because Python told us to use that

variable 23 spam starts with the number

pound sign starts and dot is not a

legitimate variable character and spam

capital spam and all cat spam are

different but this is not something that

you want to sort of depend on too much

so that's just the rule names we tend to

start them with a letter and then use

letters numbers and underscores

underscores other than the first

character are generally pretty common

and you'll see those used a lot now when

we're choosing variable names one of the

things about variables is we get to

choose the name we get to choose the

name X choose the name Y and so

sometimes we like them short but

sometimes we want them descriptive and

the notion that of making variables

descriptive is often confusing to

beginning students sometimes it's really

helpful to if you're gonna have a line

of text and you main the variable line

that's great because the next person

reading your program says oh that must

be the line of text where is it also

can't become misleading that line the

name of a variable somehow has meaning

so sometimes we'll having seen

variables and plural variables like

friend and friends like his is plural

does Python know about singular and perl

plural and the answer is no so sometimes

we pick variables that make no sense

sometimes we pick variables that make a

lot of sense this is just something that

you as a beginning programmer are going

to have to understand that we can pick

anything we want and so you'll see I'll

try to call attention this in the first

few lectures as we go through so here's

a bit of code with an assignment

statement - assignment statements of

multiplication and a print statement and

you can say what is this doing now

python is perfectly happy with this code

because it assigns it in there you have

said please go give me this as a label

and then we assign two variables and

then we're carefully pulling these two

variables back out multiplying them

together and sticking them into yet

another variable and then printing that

variable out that seems like you know we

can figure out what it is you just have

to look really careful and a single

character mistake and python is going to

be you know pretty unhappy okay so

that's one way to write this program

it's hard though because you could any

of those characters or long variables

and they're random stuff it's not very

friendly to anyone who might read your

program now this looks a little

friendlier it's the same program because

python just wants a correspondence you

picked a you picked B and you pick C and

it's really much easier for us to see

what's going on and and so this is in a

way going from here to here is much

friendlier but we can be even friendlier

if we pick mnemonic variable names so

this is this is not mnemonic this is

short and convenient this is long and

inconvenient Python is happy with any of

these here on the other hand is another

version of the exact same program and

now you think to yourself oh yeah now I

get it

35 is the number of hours $12.50 is the

rate and then we're going to multiply

the hours and the rate and come up with

a pay and we're printing out to pay now

whoever wrote this program is much is

helping us greatly understand what's

going on and that's good

choosing variable names a Python again

all three of these are the same two

Python

using variable names in a way that help

your reader understand what's going on

is a great thing the problem is the

danger is if you read this and you think

that somehow python understands payroll

that if you name a variable hours that

Python knows what hours means the answer

is Python really doesn't care what you

name the variable as long as what you

name it you use it right and so you got

to be careful and so you'll see I will

die when I write my code in these first

few weeks first few lectures I will

sometimes write it with gibberish I'll

sometimes write it with extremely short

but meaningless variable names and

sometimes I'll use meaning full variable

names and I'll call your attention to it

and and it will get you you'll start

when you look at this third kind it has

meaningful meaningful variables or

mnemonic variable names you'll just

instinctively want to give Python more

intelligence than it sort of deserves I

guess that's probably the best way to

say that so we've talked about constants

we talked about reserved words we talked

about variables and so here we have a

sentence like we've already done some of

these things where we set x equals 2 we

retrieve the old value of x and add 2 to

it so that becomes 4 and then we print 4

out print is a function that's built in

and we pass in whatever we want to print

out so this parenthesis is part of a

function call ok so an assignment

statement you have to really get it your

head around the notion that it has this

arrow nature and that it valuates this

entire right hand side before we change

the left-hand side and so you can think

of this sort of as at time step 1 it

does this and then a time step 2 it does

the copy and that's how you can have

something like X on both sides of

assignment statement and so if for

example we have X and X has 0.6 in it X

has 0.6 in it what happens is is that at

first it sort of ignores this part right

here and evaluates the expression so it

pulls the 0.6 everywhere X appears it

pulls 0.6 out

starts running these calculations and

then it has the new Vera value after all

the calculations are done then and only

then is it going to put that back into X

and so it sort of takes that and puts it

back into X and then wipes out the old

value at this point this has all been

taken care of and it's been reduced down

to the zero point nine three and so that

is what's put in as the new value so up

next we'll talk a little bit more about

making more complex expressions

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