Symbol Review

t's time to review the symbols and Python words you know and to try to pick up a few more for the next few lessons. I have written out all the Python symbols and keywords that are important to know.

In this lesson take each keyword and first try to write out what it does from memory. Next, search online for it and see what it really does. This may be difficult because some of these are difficult to search for, but try anyway.

If you get one of these wrong from memory, make an index card with the correct definition and try to "correct" your memory.

Finally, use each of these in a small Python program, or as many as you can get done. The goal is to find out what the symbol does, make sure you got it right, correct it if you do not, then use it to lock it in.

Keywords

KEYWORD	DESCRIPTION	EXAMPLE
and	Logical and.	True and False == False
as	Part of the with-as statement.	with X as Y: pass
assert	Assert (ensure) that something is true.	assert False, "Error!"
break	Stop this loop right now.	while True: break
class	Define a class.	class Person(object)
continue	Don't process more of the loop, do it again.	while True: continue
def	Define a function.	def X(): pass
del	Delete from dictionary.	del X[Y]
elif	Else if condition.	if: X; elif: Y; else: J
else	Else condition.	if: X; elif: Y; else: J
except	If an exception happens, do this.	except ValueError, e: print e
exec	Run a string as Python.	exec 'print "hello"'
finally	Exceptions or not, finally do this no matter what.	finally: pass
for	Loop over a collection of things.	for X in Y: pass
from	Importing specific parts of a module.	import X from Y
global	Declare that you want a global variable.	global X
if	If condition.	if: X; elif: Y; else: J
import	Import a module into this one to use.	import os
in	Part of for-loops. Also a test of X in Y.	for X in Y: pass also 1 in [1] == True
is	Like == to test equality.	1 is 1 == True
lambda	Create a short anonymous function.	s = lambda y: y ** y; s(3)
not	Logical not.	not True == False
or	Logical or.	True or False == True
pass	This block is empty.	def empty(): pass
print	Print this string.	print 'this string'
raise	Raise an exception when things go wrong.	raise ValueError("No")
return	Exit the function with a return value.	def X(): return Y
try	Try this block, and if exception, go to except.	try: pass
while	While loop.	while X: pass
with	With an expression as a variable do.	with X as Y: pass
yield	Pause here and return to caller.	def X(): yield Y; X().next()

Data Types

For data types, write out what makes up each one. For example, with strings write out how you create a string. For numbers write out a few numbers.

TYPE	DESCRIPTION	EXAMPLE
True	True boolean value.	True or False == True
False	False boolean value.	False and True == False
None	Represents "nothing" or "no value".	x = None
strings	Stores textual information.	x = "hello"
numbers	Stores integers.	i = 100
floats	Stores decimals.	i = 10.389
lists	Stores a list of things.	j = [1,2,3,4]
dicts	Stores a key=value mapping of things.	e = {'x': 1, 'y': 2}

String Escape Sequences

For string escape sequences, use them in strings to make sure they do what you think they do.

ESCAPE	DESCRIPTION
\\	Backslash
\'	Single-quote
\"	Double-quote
\a	Bell
\b	Backspace
\f	Formfeed
\n	Newline
\r	Carriage
\t	Tab
\v	Vertical tab

String Formats

Same thing for string formats: use them in some strings to know what they do.

ESCAPE	DESCRIPTION	EXAMPLE
%d	Decimal integers (not floating point).	"%d" % 45 == '45'
%i	Same as %d.	"%i" % 45 == '45'
%o	Octal number.	"%o" % 1000 == '1750'
%u	Unsigned decimal.	"%u" % -1000 == '-1000'
%x	Hexadecimal lowercase.	"%x" % 1000 == '3e8'
%X	Hexadecimal uppercase.	"%X" % 1000 == '3E8'
%e	Exponential notation, lowercase 'e'.	"%e" % 1000 == '1.000000e+03'
%E	Exponential notation, uppercase 'E'.	"%E" % 1000 == '1.000000E+03'
%f	Floating point real number.	"%f" % 10.34 == '10.340000'
%F	Same as %f.	"%F" % 10.34 == '10.340000'
%g	Either %f or %e, whichever is shorter.	"%g" % 10.34 == '10.34'
%G	Same as %g but uppercase.	"%G" % 10.34 == '10.34'
%c	Character format.	"%c" % 34 == '"'
%r	Repr format (debugging format).	"%r" % int == "<type 'int'>"
%s	String format.	"%s there" % 'hi' == 'hi there'
%%	A percent sign.	"%g%%" % 10.34 == '10.34%'

Operators

Some of these may be unfamiliar to you, but look them up anyway. Find out what they do, and if you still can't figure it out, save it for later.

OPERATOR	DESCRIPTION	EXAMPLE
+	Addition	2 + 4 == 6
-	Subtraction	2 - 4 == -2
*	Multiplication	2 * 4 == 8
**	Power of	2 ** 4 == 16
/	Division	2 / 4.0 == 0.5
//	Floor division	2 // 4.0 == 0.0
%	String interpolate or modulus	2 % 4 == 2
<	Less than	4 < 4 == False
>	Greater than	4 > 4 == False
<=	Less than equal	4 <= 4 == True
>=	Greater than equal	4 >= 4 == True
==	Equal	4 == 5 == False
!=	Not equal	4 != 5 == True
<>	Not equal	4 <> 5 == True
( )	Parenthesis	len('hi') == 2
[ ]	List brackets	[1,3,4]
{ }	Dict curly braces	{'x': 5, 'y': 10}
@	At (decorators)	@classmethod
,	Comma	range(0, 10)
:	Colon	def X():
.	Dot	self.x = 10
=	Assign equal	x = 10
;	semi-colon	print "hi"; print "there"
+=	Add and assign	x = 1; x += 2
-=	Subtract and assign	x = 1; x -= 2
*=	Multiply and assign	x = 1; x *= 2
/=	Divide and assign	x = 1; x /= 2
//=	Floor divide and assign	x = 1; x //= 2
%=	Modulus assign	x = 1; x %= 2
**=	Power assign	x = 1; x **= 2