Download Python Quick review

Python Quick review
CS360
Dick Steflik
Starting with Hello World
• start the python shell type:
print "Hello World!"
or
put into file hello.py with text editor
restart the python shell and type:
import hello
or
at OS command prompt type python hello.py
Variables, Expressions and Statements
• type(<expression>)
print <expr>, ..., <expr>
<variable> = <expression>
+-*/
operators: ** %
() > ** > * / % > + -
returns the type of the expression
print several expressions
assign value to variable operators:
normal arithmetic operators
** is exponentiation, % is mod
precedence (otherwise mostly
left-to-right)
<var> = raw_input(<prompt>) read a text line of input
<var> = input(<prompt>) read a numerical line of input
# text
comment
Functions
• int(<float or string>)
str(<nonstring expression>)
import math
math.sin, math.cos, math.sqrt
help('math')
convert to int
convert to string
make math functions avail
examples of math functions
list of all math functions
(q to quit help)
defining functions:
def <name>(<param>, ..., <param>):
<statements>
return statement
(immediately exits function,
returning a value):
return <expression>
Conditionals
• type boolean (logical tests): either True or False
x != y
# x is not equal to y
x>y
# x is greater than y
x<y
# x is less than y
x >= y
# x is greater than or equal to y
x <= y
# x is less than or equal to y
x and y
# x and y are both true
x or y
# x or y (or both) are true
not x
# x is not true
if/else statements
• if <test>:
<statements>
if <test>:
<statements>
else:
<statements>
if <test1>:
<statements>
elif <test2>:
<statements>
else:
<statements>
Strings
• strings are immutable; to change one construct a new one
s[index]
return character at index (0 based)
len(s)
length of string
s[-1], s[-2], s[-3]
s[i:j]
s[:j]
s[i:]
<string> + <string>
help('str')
s.lower()
str.isalpha
last character, 2nd to last, 3rd to last
slice of string: i through (j - 1)
slice from beginning through (j - 1)
slice from i to end of string
concatenate two strings
show string functions (q to quit help)
returns lowercase version of s
is alphabetic character (boolean function)
foreach loop over string:
for <variable> in <string>:
<statements>
Lists
• [<expr>, ..., <expr>] a list
same use of [], len
range(n)
range(i, j)
[]
<expression> in <list>
<list> + <list>
list1 = list(list2)
<var> = list(<string>)
<list var>.append(<expr>)
<list var>.remove(<expr>)
del lst[i]
foreach loop over list:
for <variable> in <list>:
<statements>
lists are very similar to strings
list of integers 0 through (n - 1)
list of integers i through (j - 1)
empty list
test for list membership (boolean)
append two lists together to form
new list
make a copy of list2, store in list1
convert string to list
append value to end of list
remove given value from list
remove value at index i
Files
• <var> = open(<name>, 'r')
<var> = open(<name>)
<file variable>.readLine()
<var> = open(<name>, ‘w')
<file variable>.write(<string>)
open a file for input
same as above, ‘r’ is default
return next line of file as a
string
return all lines of file as list
of strings
open a file for output
write <string> to the file
<file variable>.close()
close the file
<file variable>.readLines()
Statements
• assignment
function calls
print
if/elseif/else
for/else
while/else
pass
break, continue
try/except/finally
raise
import,from
def,return,yield
a,b,c = 1,2,3
log.write(“some text\n”)
print(“some fun\n”)
conditional execution
fixed iteration
indeterminate iteration
empty statement (no-op)
force exit a loop
catching exceptions
trigger an exception
make members of a module accessible
defining functions
Statements (cont.)
• class
global
del
exec
assert
with/as
Defining objects
namespaces
deleting references
running code strings, like interpreting a string
containing python code
debugging your code
context managers