02 - Expressions A - Python for Everybody Course
Summary
TLDRThis 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
📚 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.
🔍 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.
Mindmap
Keywords
💡Constants
💡Reserved Words
💡Variables
💡Assignment Statement
💡Syntax
💡Conditional Execution
💡Mnemonic Variable Names
💡Function
💡Expression
💡Variable Naming Conventions
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
[Music]
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