A comment is like a note-to-self that you include
directly in a program as a way to explain something to yourself
for later, or to someone else who reads your program.
# This is a comment
# This is one too, but not recommended
print('Something') # And so is this
Let's explain:
A comment begins with a
#
and ends at the end of the line.
Anything written as a comment is not treated by your
computer as a "command" or as programming intent.
Thus, as far as programming goes, the above program is as
good as:
print('Something')
You can insert comments in various places in a program:
1.1 Exercise:
Under your
unit0,
create a
module1
folder (if you haven't already), and then write up the
above in
my_comments.py.
Fix the second comment to start at the beginning of the line,
and add an entirely new comment line of your own.
Sometimes one needs a comment to spill over multiple lines,
as in
# I wrote this program at midnight
20 seconds before the deadline
print('Something')
Notice the missing
#
in the second line of the comment.
1.2 Exercise:
Write up the above in
comment_error.py.
Remember to save the file (and do this for every exercise in
the future that involves a program).
What is the error displayed when you try to run the program?
Write the error in your
module1.pdf
file.
1.1 Whitespace
Consider the following program:
print ( 'Hello World!' )
Notice the spaces inserted in various places.
1.3 Exercise:
Write up the above in
whitespace_example.py.
Does the program run?
Write your answer in
module1.pdf
file. Remember: if it's not clear where to write your answer,
write it in your module pdf
(for this module that's module1.pdf)
Consider this variation
print('Hello World!')
(Two spaces before
print).
1.4 Exercise:
Write up the above in
whitespace_example2.py.
What is the error produced?
Finally, look at:
print('Hello World!')
(Extra spaces between Hello and World.)
1.5 Exercise:
Write up the above in
whitespace_example3.py.
What is printed out?
Write your answer in
module1.pdf
Let's point out a few things:
Some kinds of whitespace, even if ill-advised, is permitted.
When starting a line of code, proper indentation is
expected, which is why we got an error when we indented the
line starting with
print.
The extra spaces between Hello and World are perfectly
acceptable if the goal is to print them. Printing accepts whatever
spaces you want printed.
1.2 Strings
A string in Python is a sequence of letters, digits, or
symbols (like $ or *) surrounded by either
A pair of double quotes, as in:
"Hello World!"
A pair of single quotes as in:
'Hello World!'
Note:
Whichever quote you use to start a string must be used to end
the string.
The ending quote must be on the same line as the starting quote.
There are special techniques to handle long strings that
need to spill over multiple lines (which we'll see below).
This raises some questions:
Is it possible to print a single line but with multiple
print statements?
How does one print a quote?
First, note that we can use single or double quotes for different
strings in the same program:
print('Hello')
print("World!")
1.6 Exercise:
Confirm that Hello and World! get printed on two lines
by writing the above in
my_string_example.py.
A print statement prints the string within parenthesis and then goes
to the next line of output, which is why we see World! on the next line.
To keep printing on the same line:
print('Hello', end=' ')
print('World!')
(Note: we've reverted to using single-quotes for all strings, which
will be our preference.)
1.7 Exercise:
Confirm by writing the above in
my_string_example2.py.
We'll now go the other way and have a single string itself contain a
directive to spill over to the next line.
print('Hello\nWorld!')
Notice the backslash
\
followed by
n
inside the string:
'Hello\nWorld!'
1.8 Exercise:
Write up the above
my_string_example3.py.
What is the output?
Write your answer in
module1.pdf
Strings can embed special so-called escape sequences
that begin with backslash.
This will give us one way to print a quote:
print('My friend\'s friend\'s dog\'s friend')
Another way is to use one set of quotes to delimit the string
that are different from the ones used within:
How does one print a backslash itself? By using a double
backslash:
print('The backslash character, \\, is less intimidating now')
1.9 Exercise:
Write a program called
practice_escaping.py
that prints out
" " \\\
" " \
"""""" \
" " \
" " \\\
1.10 video:
Another use of backslash: to make long strings
Sometimes we need to type in a really long string.
The following does NOT work:
print('An Ogden Nash poem:')
print('The camel has a single hump;
The dromedary, two;
Or else the other way around.
I’m never sure. Are you?')
1.11 Exercise:
Write a program called
my_string_example4.py
with the above program and run it. What is the error you observe?
Write your answer in
module1.pdf
To spread a single string over multiple lines, one uses
a triple quote as in:
print('An Ogden Nash poem:')
print('''The camel has a single hump;
The dromedary, two;
Or else the other way around.
I’m never sure. Are you?''')
1.12 Exercise:
Write a program called
my_string_example5.py
with a 5-line limerick.
Empty strings:
It is possible to not have anything in a string, as in:
print('')
Notice: there are no letters, digits or anything between the
two single quotes above.
Such a string is called an empty string.
Odd as it may seem, empty strings are useful (we'll see
later) when you want to add strings to make a longer string.
1.3 Case sensitivity
What if we had used uppercase P instead of lowercase p in print?
Print('Hello World!')
1.13 Exercise:
Write up the above program in
case_error.py
and run it. What is the error you get?
Write your answer in
module1.pdf
What if we changed the case inside a string?
print('helLo WoRLd!')
1.14 Exercise:
Write up the above program in
my_string_example6.py
and run it to see if it works.
Python is case sensitive but strings are like data inside
programs, which means they can be whatever we like.
The two strings
'Hello World!'
and
'helLo WoRLd!'
are fine as two different strings, if that's we want.
However, Python has only one
print
and so it won't recognize
Print (with capital P).
1.4 A peek at the future
We will occasionally jump ahead and present an example
of a program that does something advanced, just so you get a feel
for what's coming.
With these examples, we'll only ask you to type up the program and
run the program.
And then we'll point out a few features, just so that you get
acquainted with features that you'll eventually encounter.
import tkinter as tk
window = tk.Tk()
canvas = tk.Canvas(master=window, width=500, height=400)
canvas.pack()
for i in range(1, 10):
fontSize = i * 20
fontStr = 'Times ' + str(fontSize) + ' italic bold'
startx = 200 + 10 * i
starty = 20 + i * 20
canvas.create_text(
startx, starty, text="Hello", font=fontStr, fill='grey')
window.mainloop()
1.15 Exercise:
Write up the above program in
hello_gui.py,
being careful to type it in exactly as shown above, paying attention
to every keystroke.
Run it to get something like:
If something went wrong with the typing, you can instead
download hello_gui.py but
only after giving the typing a good try.
Now let's point out a few features of the program, and we'll
focus on the middle section:
Our belief is that by showing you bits and pieces of advanced code,
you'll be primed to absorb these concepts when we work through them.
One of the most important things to observe about is
that some code is indented:
1.5 A reminder about computerese
As you complete Module 1 and ready that for submission you
might want to review:
Did you make your
module1
folder? Did you ensure that the
m
in
module1
is lowercase and that there's no space
module
and
1?
Are your Python programs from this module in that folder?
Did you make your
module1.pdf
in this folder? Does it have the non-coding questions answered within?
Do you recall how to make a zip?
If it's a bit hazy, please make sure to review from
the material on
computerese from Module 0.
