2. Uses of Python
shell tools
system admin tools, command line programs
extension-language work
rapid prototyping and development
language-based modules
instead of special-purpose parsers
graphical user interfaces
database access
distributed programming
Internet scripting
3. Basic operations
Assignment:
size = 40
a = b = c = 3
Numbers
integer, float
complex numbers: 1j+3, abs(z)
Strings
'hello world', 'it's hot'
"bye world"
continuation via or use """ long text """"
4. String operations
concatenate with + or neighbors
word = 'Help' + x
word = 'Help' 'a'
subscripting of strings
'Hello'[2] 'l'
slice: 'Hello'[1:2] 'el'
word[-1] last character
len(word) 5
immutable: cannot assign to subscript
5. Lists
lists can be heterogeneous
a = ['spam', 'eggs', 100, 1234, 2*2]
Lists can be indexed and sliced:
a[0] spam
a[:2] ['spam', 'eggs']
Lists can be manipulated
a[2] = a[2] + 23
a[0:2] = [1,12]
a[0:0] = []
len(a) 5
6. Basic programming
a,b = 0, 1
# non-zero = true
while b < 10:
# formatted output, without
n
print b,
# multiple assignment
a,b = b, a+b
7. Control flow: if
x = int(raw_input("Please enter #:"))
if x < 0:
x = 0
print 'Negative changed to zero'
elif x == 0:
print 'Zero'
elif x == 1:
print 'Single'
else:
print 'More'
no case statement
8. Control flow: for
a = ['cat', 'window', 'defenestrate']
for x in a:
print x, len(x)
no arithmetic progression, but
range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8,
9]
for i in range(len(a)):
print i, a[i]
do not modify the sequence being
iterated over
9. Loops: break, continue, else
break and continue like C
else after loop exhaustion
for n in range(2,10):
for x in range(2,n):
if n % x == 0:
print n, 'equals', x, '*', n/x
break
else:
# loop fell through without finding a
factor
print n, 'is prime'
10. Defining functions
def fib(n):
"""Print a Fibonacci series up to n."""
a, b = 0, 1
while b < n:
print b,
a, b = b, a+b
>>> fib(2000)
First line is docstring
first look for variables in local, then global
need global to assign global variables
11. Functions: default argument
values
def ask_ok(prompt, retries=4,
complaint='Yes or no, please!'):
while 1:
ok = raw_input(prompt)
if ok in ('y', 'ye', 'yes'): return 1
if ok in ('n', 'no'): return 0
retries = retries - 1
if retries < 0: raise IOError,
'refusenik error'
print complaint
>>> ask_ok('Really?')
12. Keyword arguments
last arguments can be given as keywords
def parrot(voltage, state='a stiff', action='voom',
type='Norwegian blue'):
print "-- This parrot wouldn't", action,
print "if you put", voltage, "Volts through it."
print "Lovely plumage, the ", type
print "-- It's", state, "!"
parrot(1000)
parrot(action='VOOOM', voltage=100000)
13. Lambda forms
anonymous functions
may not work in older versions
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
f(1)
14. List methods
append(x)
extend(L)
append all items in list (like Tcl lappend)
insert(i,x)
remove(x)
pop([i]), pop()
create stack (FIFO), or queue (LIFO) pop(0)
index(x)
return the index for value x
15. List methods
count(x)
how many times x appears in list
sort()
sort items in place
reverse()
reverse list
16. Functional programming tools
filter(function, sequence)
def f(x): return x%2 != 0 and x%3 0
filter(f, range(2,25))
map(function, sequence)
call function for each item
return list of return values
reduce(function, sequence)
return a single value
call binary function on the first two items
then on the result and next item
iterate
17. List comprehensions (2.0)
Create lists without map(), filter(), lambda
= expression followed by for clause + zero or more for or
of clauses
>>> vec = [2,4,6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [{x: x**2} for x in vec}
[{2: 4}, {4: 16}, {6: 36}]
18. List comprehensions
cross products:
>>> vec1 = [2,4,6]
>>> vec2 = [4,3,-9]
>>> [x*y for x in vec1 for y in vec2]
[8,6,-18, 16,12,-36, 24,18,-54]
>>> [x+y for x in vec1 and y in vec2]
[6,5,-7,8,7,-5,10,9,-3]
>>> [vec1[i]*vec2[i] for i in
range(len(vec1))]
[8,12,-54]
19. List comprehensions
can also use if:
>>> [3*x for x in vec if x >
3]
[12, 18]
>>> [3*x for x in vec if x <
2]
[]
20. del – removing list items
remove by index, not value
remove slices from list (rather than by
assigning an empty list)
>>> a = [-1,1,66.6,333,333,1234.5]
>>> del a[0]
>>> a
[1,66.6,333,333,1234.5]
>>> del a[2:4]
>>> a
[1,66.6,1234.5]
21. Tuples and sequences
lists, strings, tuples: examples of sequence type
tuple = values separated by commas
>>> t = 123, 543, 'bar'
>>> t[0]
123
>>> t
(123, 543, 'bar')
22. Tuples
Tuples may be nested
>>> u = t, (1,2)
>>> u
((123, 542, 'bar'), (1,2))
kind of like structs, but no element
names:
(x,y) coordinates
database records
like strings, immutable can't assign to
individual items
24. Tuples
sequence unpacking distribute elements across
variables
>>> t = 123, 543, 'bar'
>>> x, y, z = t
>>> x
123
packing always creates tuple
unpacking works for any sequence
25. Dictionaries
like Tcl or awk associative arrays
indexed by keys
keys are any immutable type: e.g., tuples
but not lists (mutable!)
uses 'key: value' notation
>>> tel = {'hgs' : 7042, 'lennox': 7018}
>>> tel['cs'] = 7000
>>> tel
26. Dictionaries
no particular order
delete elements with del
>>> del tel['foo']
keys() method unsorted list of keys
>>> tel.keys()
['cs', 'lennox', 'hgs']
use has_key() to check for existence
>>> tel.has_key('foo')
0
27. Conditions
can check for sequence membership
with is and is not:
>>> if (4 in vec):
... print '4 is'
chained comparisons: a less than b
AND b equals c:
a < b == c
and and or are short-circuit operators:
evaluated from left to right
stop evaluation as soon as outcome clear
28. Conditions
Can assign comparison to variable:
>>> s1,s2,s3='', 'foo', 'bar'
>>> non_null = s1 or s2 or s3
>>> non_null
foo
Unlike C, no assignment within expression
30. Modules
collection of functions and variables, typically in scripts
definitions can be imported
file name is module name + .py
e.g., create module fibo.py
def fib(n): # write Fib. series up to n
...
def fib2(n): # return Fib. series up to n
31. Modules
import module:
import fibo
Use modules via "name space":
>>> fibo.fib(1000)
>>> fibo.__name__
'fibo'
can give it a local name:
>>> fib = fibo.fib
>>> fib(500)
32. Modules
function definition + executable
statements
executed only when module is imported
modules have private symbol tables
avoids name clash for global variables
accessible as module.globalname
can import into name space:
>>> from fibo import fib, fib2
>>> fib(500)
can import all names defined by module:
>>> from fibo import *
33. Module search path
current directory
list of directories specified in PYTHONPATH
environment variable
uses installation-default if not defined,
e.g., .:/usr/local/lib/python
uses sys.path
>>> import sys
>>> sys.path
['', 'C:PROGRA~1Python2.2', 'C:Program Files
Python2.2DLLs', 'C:Program FilesPython2.2lib', 'C:
Program FilesPython2.2liblib-tk', 'C:Program Files
Python2.2', 'C:Program FilesPython2.2libsite-
packages']
34. Classes
mixture of C++ and Modula-3
multiple base classes
derived class can override any methods of
its base class(es)
method can call the method of a base
class with the same name
objects have private data
C++ terms:
all class members are public
all member functions are virtual
no constructors or destructors (not needed)
35. Classes
classes (and data types) are objects
built-in types cannot be used as base classes by user
arithmetic operators, subscripting can be redefined for
class instances (like C++, unlike Java)
37. Namespaces
mapping from name to object:
built-in names (abs())
global names in module
local names in function invocation
attributes = any following a dot
z.real, z.imag
attributes read-only or writable
module attributes are writeable
38. Namespaces
scope = textual region of Python program
where a namespace is directly accessible
(without dot)
innermost scope (first) = local names
middle scope = current module's global names
outermost scope (last) = built-in names
assignments always affect innermost
scope
don't copy, just create name bindings to
objects
global indicates name is in global scope
39. Class objects
obj.name references (plus module!):
class MyClass:
"A simple example class"
i = 123
def f(self):
return 'hello world'
>>> MyClass.i
123
MyClass.f is method object
40. Class objects
class instantiation:
>>> x = MyClass()
>>> x.f()
'hello world'
creates new instance of class
note x = MyClass vs. x = MyClass()
___init__() special method for
initialization of object
def __init__(self,realpart,imagpart):
self.r = realpart
self.i = imagpart
41. Instance objects
attribute references
data attributes (C++/Java data members)
created dynamically
x.counter = 1
while x.counter < 10:
x.counter = x.counter * 2
print x.counter
del x.counter
42. Method objects
Called immediately:
x.f()
can be referenced:
xf = x.f
while 1:
print xf()
object is passed as first argument of function 'self'
x.f() is equivalent to MyClass.f(x)
45. Private variables
No real support, but textual replacement (name
mangling)
__var is replaced by _classname_var
prevents only accidental modification, not true
protection
47. Handling exceptions
while 1:
try:
x = int(raw_input("Please enter a number: "))
break
except ValueError:
print "Not a valid number"
First, execute try clause
if no exception, skip except clause
if exception, skip rest of try clause and use
except clause
if no matching exception, attempt outer try
statement
48. Handling exceptions
try.py
import sys
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'lines:',
len(f.readlines())
f.close
e.g., as python try.py *.py
![String operations
concatenate with + or neighbors
word = 'Help' + x
word = 'Help' 'a'
subscripting of strings
'Hello'[2] 'l'
slice: 'Hello'[1:2] 'el'
word[-1] last character
len(word) 5
immutable: cannot assign to subscript](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-4-320.jpg)
![Lists
lists can be heterogeneous
a = ['spam', 'eggs', 100, 1234, 2*2]
Lists can be indexed and sliced:
a[0] spam
a[:2] ['spam', 'eggs']
Lists can be manipulated
a[2] = a[2] + 23
a[0:2] = [1,12]
a[0:0] = []
len(a) 5](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-5-320.jpg)
![Control flow: for
a = ['cat', 'window', 'defenestrate']
for x in a:
print x, len(x)
no arithmetic progression, but
range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8,
9]
for i in range(len(a)):
print i, a[i]
do not modify the sequence being
iterated over](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-8-320.jpg)
![List methods
append(x)
extend(L)
append all items in list (like Tcl lappend)
insert(i,x)
remove(x)
pop([i]), pop()
create stack (FIFO), or queue (LIFO) pop(0)
index(x)
return the index for value x](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-14-320.jpg)
![List comprehensions (2.0)
Create lists without map(), filter(), lambda
= expression followed by for clause + zero or more for or
of clauses
>>> vec = [2,4,6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [{x: x**2} for x in vec}
[{2: 4}, {4: 16}, {6: 36}]](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-17-320.jpg)
![List comprehensions
cross products:
>>> vec1 = [2,4,6]
>>> vec2 = [4,3,-9]
>>> [x*y for x in vec1 for y in vec2]
[8,6,-18, 16,12,-36, 24,18,-54]
>>> [x+y for x in vec1 and y in vec2]
[6,5,-7,8,7,-5,10,9,-3]
>>> [vec1[i]*vec2[i] for i in
range(len(vec1))]
[8,12,-54]](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-18-320.jpg)
![List comprehensions
can also use if:
>>> [3*x for x in vec if x >
3]
[12, 18]
>>> [3*x for x in vec if x <
2]
[]](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-19-320.jpg)
![del – removing list items
remove by index, not value
remove slices from list (rather than by
assigning an empty list)
>>> a = [-1,1,66.6,333,333,1234.5]
>>> del a[0]
>>> a
[1,66.6,333,333,1234.5]
>>> del a[2:4]
>>> a
[1,66.6,1234.5]](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-20-320.jpg)
![Tuples and sequences
lists, strings, tuples: examples of sequence type
tuple = values separated by commas
>>> t = 123, 543, 'bar'
>>> t[0]
123
>>> t
(123, 543, 'bar')](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-21-320.jpg)
![Dictionaries
like Tcl or awk associative arrays
indexed by keys
keys are any immutable type: e.g., tuples
but not lists (mutable!)
uses 'key: value' notation
>>> tel = {'hgs' : 7042, 'lennox': 7018}
>>> tel['cs'] = 7000
>>> tel](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-25-320.jpg)
![Dictionaries
no particular order
delete elements with del
>>> del tel['foo']
keys() method unsorted list of keys
>>> tel.keys()
['cs', 'lennox', 'hgs']
use has_key() to check for existence
>>> tel.has_key('foo')
0](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-26-320.jpg)
![Comparing sequences
(1,2,3) < (1,2,4)
[1,2,3] < [1,2,4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1,2,3) == (1.0,2.0,3.0)
(1,2) < (1,2,-1)](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-29-320.jpg)
![Module search path
current directory
list of directories specified in PYTHONPATH
environment variable
uses installation-default if not defined,
e.g., .:/usr/local/lib/python
uses sys.path
>>> import sys
>>> sys.path
['', 'C:PROGRA~1Python2.2', 'C:Program Files
Python2.2DLLs', 'C:Program FilesPython2.2lib', 'C:
Program FilesPython2.2liblib-tk', 'C:Program Files
Python2.2', 'C:Program FilesPython2.2libsite-
packages']](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-33-320.jpg)
![Handling exceptions
try.py
import sys
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'lines:',
len(f.readlines())
f.close
e.g., as python try.py *.py](https://image.slidesharecdn.com/pythonprogramming-250318053525-95f739b0/85/python-language-programming-presentation-48-320.jpg)