python

Август 12, 2022

Содержание

2. Advanced Programming Spring 2002 Introduction Most recent popular (scripting/extension) language although origin ~1991 heritage: teaching language
3. Advanced Programming Spring 2002 Python philosophy Coherence not hard to read, write and maintain power scope
4. Advanced Programming Spring 2002 Python features Lutz, Programming Python
5. Advanced Programming Spring 2002 Python features Lutz, Programming Python
6. Advanced Programming Spring 2002 Python elements from C++, Modula-3 (modules), ABC, Icon (slicing) same family as
7. Advanced Programming Spring 2002 Uses of Python shell tools system admin tools, command line programs extension-language
8. Advanced Programming Spring 2002 What not to use Python (and kin) for most scripting languages share
9. Advanced Programming Spring 2002 Using python /usr/local/bin/python #! /usr/bin/env python interactive use Python 1.6 (#1, Sep
10. Advanced Programming Spring 2002 Python structure modules: Python source files or C extensions import, top-level via
11. Advanced Programming Spring 2002 First example #!/usr/local/bin/python # import systems module import sys marker = '::::::'
12. Advanced Programming Spring 2002 Basic operations Assignment: size = 40 a = b = c =
13. Advanced Programming Spring 2002 String operations concatenate with + or neighbors word = 'Help' + x
14. Advanced Programming Spring 2002 Lists lists can be heterogeneous a = ['spam', 'eggs', 100, 1234, 2*2]
15. Advanced Programming Spring 2002 Basic programming a,b = 0, 1 # non-zero = true while b
16. Advanced Programming Spring 2002 Control flow: if x = int(raw_input("Please enter #:")) if x x =
17. Advanced Programming Spring 2002 Control flow: for a = ['cat', 'window', 'defenestrate'] for x in a:
18. Advanced Programming Spring 2002 Loops: break, continue, else break and continue like C else after loop
19. Advanced Programming Spring 2002 Do nothing pass does nothing syntactic filler while 1: pass
20. Advanced Programming Spring 2002 Defining functions def fib(n): """Print a Fibonacci series up to n.""" a,
21. Advanced Programming Spring 2002 Functions: default argument values def ask_ok(prompt, retries=4, complaint='Yes or no, please!'): while
22. Advanced Programming Spring 2002 Keyword arguments last arguments can be given as keywords def parrot(voltage, state='a
23. Advanced Programming Spring 2002 Lambda forms anonymous functions may not work in older versions def make_incrementor(n):
24. Advanced Programming Spring 2002 List methods append(x) extend(L) append all items in list (like Tcl lappend)
25. Advanced Programming Spring 2002 List methods count(x) how many times x appears in list sort() sort
26. Advanced Programming Spring 2002 Functional programming tools filter(function, sequence) def f(x): return x%2 != 0 and
27. Advanced Programming Spring 2002 List comprehensions (2.0) Create lists without map(), filter(), lambda = expression followed
28. Advanced Programming Spring 2002 List comprehensions cross products: >>> vec1 = [2,4,6] >>> vec2 = [4,3,-9]
29. Advanced Programming Spring 2002 List comprehensions can also use if: >>> [3*x for x in vec
30. Advanced Programming Spring 2002 del – removing list items remove by index, not value remove slices
31. Advanced Programming Spring 2002 Tuples and sequences lists, strings, tuples: examples of sequence type tuple =
32. Advanced Programming Spring 2002 Tuples Tuples may be nested >>> u = t, (1,2) >>> u
33. Advanced Programming Spring 2002 Tuples Empty tuples: () >>> empty = () >>> len(empty) 0 one
34. Advanced Programming Spring 2002 Tuples sequence unpacking ? distribute elements across variables >>> t = 123,
35. Advanced Programming Spring 2002 Dictionaries like Tcl or awk associative arrays indexed by keys keys are
36. Advanced Programming Spring 2002 Dictionaries no particular order delete elements with del >>> del tel['foo'] keys()
37. Advanced Programming Spring 2002 Conditions can check for sequence membership with is and is not: >>>
38. Advanced Programming Spring 2002 Conditions Can assign comparison to variable: >>> s1,s2,s3='', 'foo', 'bar' >>> non_null
39. Advanced Programming Spring 2002 Comparing sequences unlike C, can compare sequences (lists, tuples, ...) lexicographical comparison:
40. Advanced Programming Spring 2002 Comparing sequences (1,2,3) [1,2,3] 'ABC' (1,2,3) == (1.0,2.0,3.0) (1,2)
41. Advanced Programming Spring 2002 Modules collection of functions and variables, typically in scripts definitions can be
42. Advanced Programming Spring 2002 Modules import module: import fibo Use modules via "name space": >>> fibo.fib(1000)
43. Advanced Programming Spring 2002 Modules function definition + executable statements executed only when module is imported
44. Advanced Programming Spring 2002 Module search path current directory list of directories specified in PYTHONPATH environment
45. Advanced Programming Spring 2002 Compiled Python files include byte-compiled version of module if there exists fibo.pyc
46. Advanced Programming Spring 2002 Standard modules system-dependent list always sys module >>> import sys >>> sys.p1
47. Advanced Programming Spring 2002 Module listing use dir() for each module >>> dir(fibo) ['___name___', 'fib', 'fib2']
48. Advanced Programming Spring 2002 Classes mixture of C++ and Modula-3 multiple base classes derived class can
49. Advanced Programming Spring 2002 Classes classes (and data types) are objects built-in types cannot be used
50. Advanced Programming Spring 2002 Class definitions Class ClassName: ... must be executed can be executed conditionally
51. Advanced Programming Spring 2002 Namespaces mapping from name to object: built-in names (abs()) global names in
52. Advanced Programming Spring 2002 Namespaces scope = textual region of Python program where a namespace is
53. Advanced Programming Spring 2002 Class objects obj.name references (plus module!): class MyClass: "A simple example class"
54. Advanced Programming Spring 2002 Class objects class instantiation: >>> x = MyClass() >>> x.f() 'hello world'
55. Advanced Programming Spring 2002 Instance objects attribute references data attributes (C++/Java data members) created dynamically x.counter
56. Advanced Programming Spring 2002 Method objects Called immediately: x.f() can be referenced: xf = x.f while
57. Advanced Programming Spring 2002 Notes on classes Data attributes override method attributes with the same name
58. Advanced Programming Spring 2002 Another example bag.py class Bag: def __init__(self): self.data = [] def add(self,
59. Advanced Programming Spring 2002 Another example, cont'd. invoke: >>> from bag import * >>> l =
60. Advanced Programming Spring 2002 Inheritance class DerivedClassName(BaseClassName) ... search class attribute, descending chain of base classes
61. Advanced Programming Spring 2002 Multiple inheritance class DerivedClass(Base1,Base2,Base3): depth-first, left-to-right problem: class derived from two classes
62. Advanced Programming Spring 2002 Private variables No real support, but textual replacement (name mangling) __var is
63. Advanced Programming Spring 2002 ~ C structs Empty class definition: class Employee: pass john = Employee()
64. Advanced Programming Spring 2002 Exceptions syntax (parsing) errors while 1 print 'Hello World' File " ",
65. Advanced Programming Spring 2002 Handling exceptions while 1: try: x = int(raw_input("Please enter a number: "))
66. Advanced Programming Spring 2002 Handling exceptions try.py import sys for arg in sys.argv[1:]: try: f =
68. Скачать презентацию

Слайд 2

Advanced Programming Spring 2002
Introduction
Most recent popular (scripting/extension) language
although origin ~1991
heritage: teaching language

(ABC)
Tcl: shell
perl: string (regex) processing
object-oriented
rather than add-on (OOTcl)

Слайд 3

Advanced Programming Spring 2002
Python philosophy
Coherence
not hard to read, write and maintain
power
scope
rapid development

+ large systems
objects
integration
hybrid systems

Слайд 4

Advanced Programming Spring 2002
Python features
Lutz, Programming Python

Слайд 5

Advanced Programming Spring 2002
Python features
Lutz, Programming Python

Слайд 6

Advanced Programming Spring 2002
Python
elements from C++, Modula-3 (modules), ABC, Icon (slicing)
same family

as Perl, Tcl, Scheme, REXX, BASIC dialects

Слайд 7

Advanced Programming Spring 2002
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

Слайд 8

Advanced Programming Spring 2002
What not to use Python (and kin) for
most scripting

languages share these
not as efficient as C
but sometimes better built-in algorithms (e.g., hashing and sorting)
delayed error notification
lack of profiling tools

Слайд 9

Advanced Programming Spring 2002
Using python
/usr/local/bin/python
#! /usr/bin/env python
interactive use
Python 1.6 (#1, Sep 24

2000, 20:40:45) [GCC 2.95.1 19990816 (release)] on sunos5
Copyright (c) 1995-2000 Corporation for National Research Initiatives.
All Rights Reserved.
Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved.
>>>
python –c command [arg] ...
python –i script
read script first, then interactive

Слайд 10

Advanced Programming Spring 2002
Python structure
modules: Python source files or C extensions
import, top-level

via from, reload
statements
control flow
create objects
indentation matters – instead of {}
objects
everything is an object
automatically reclaimed when no longer needed

Слайд 11

Advanced Programming Spring 2002
First example
#!/usr/local/bin/python
# import systems module
import sys
marker = '::::::'
for

name in sys.argv[1:]:
input = open(name, 'r')
print marker + name
print input.read()

Слайд 12

Advanced Programming Spring 2002
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 """"

Слайд 13

Advanced Programming Spring 2002
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

Слайд 14

Advanced Programming Spring 2002
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

Слайд 15

Advanced Programming Spring 2002
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

Слайд 16

Advanced Programming Spring 2002
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

Слайд 17

Advanced Programming Spring 2002
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

Слайд 18

Advanced Programming Spring 2002
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'

Слайд 19

Advanced Programming Spring 2002
Do nothing
pass does nothing
syntactic filler
while 1:
pass

Слайд 20

Advanced Programming Spring 2002
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

Слайд 21

Advanced Programming Spring 2002
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?')

Слайд 22

Advanced Programming Spring 2002
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)

Слайд 23

Advanced Programming Spring 2002
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)

Слайд 24

Advanced Programming Spring 2002
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

Слайд 25

Advanced Programming Spring 2002
List methods
count(x)
how many times x appears in list
sort()
sort items

in place
reverse()
reverse list

Слайд 26

Advanced Programming Spring 2002
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

Слайд 27

Advanced Programming Spring 2002
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}]

Слайд 28

Advanced Programming Spring 2002
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]

Слайд 29

Advanced Programming Spring 2002
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]
[]

Слайд 30

Advanced Programming Spring 2002
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]

Слайд 31

Advanced Programming Spring 2002
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')

Слайд 32

Advanced Programming Spring 2002
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

Слайд 33

Advanced Programming Spring 2002
Tuples
Empty tuples: ()
>>> empty = ()
>>> len(empty)
0
one item ?

trailing comma
>>> singleton = 'foo',

Слайд 34

Advanced Programming Spring 2002
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

Слайд 35

Advanced Programming Spring 2002
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

Слайд 36

Advanced Programming Spring 2002
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

Слайд 37

Advanced Programming Spring 2002
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

Слайд 38

Advanced Programming Spring 2002
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

Слайд 39

Advanced Programming Spring 2002
Comparing sequences
unlike C, can compare sequences (lists, tuples, ...)
lexicographical

comparison:
compare first; if different ? outcome
continue recursively
subsequences are smaller
strings use ASCII comparison
can compare objects of different type, but by type name (list < string < tuple)

Слайд 40

Advanced Programming Spring 2002
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)

Слайд 41

Advanced Programming Spring 2002
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

Слайд 42

Advanced Programming Spring 2002
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)

Слайд 43

Advanced Programming Spring 2002
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 *

Слайд 44

Advanced Programming Spring 2002
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~1\\Python2.2', 'C:\\Program Files\\Python2.2\\DLLs', 'C:\\Program Files\\Python2.2\\lib', 'C:\\Program Files\\Python2.2\\lib\\lib-tk', 'C:\\Program Files\\Python2.2', 'C:\\Program Files\\Python2.2\\lib\\site-packages']

Слайд 45

Advanced Programming Spring 2002
Compiled Python files
include byte-compiled version of module if there

exists fibo.pyc in same directory as fibo.py
only if creation time of fibo.pyc matches fibo.py
automatically write compiled file, if possible
platform independent
doesn't run any faster, but loads faster
can have only .pyc file ? hide source

Слайд 46

Advanced Programming Spring 2002
Standard modules
system-dependent list
always sys module
>>> import sys
>>> sys.p1
'>>> '
>>>

sys.p2
'... '
>>> sys.path.append('/some/directory')

Слайд 47

Advanced Programming Spring 2002
Module listing
use dir() for each module
>>> dir(fibo)
['_name_', 'fib', 'fib2']
>>>

dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__', '__st
din__', '__stdout__', '_getframe', 'argv', 'builtin_module_names', 'byteorder',
'copyright', 'displayhook', 'dllhandle', 'exc_info', 'exc_type', 'excepthook', '
exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getrecursionlimit', '
getrefcount', 'hexversion', 'last_type', 'last_value', 'maxint', 'maxunicode', '
modules', 'path', 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setpr
ofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'version',
'version_info', 'warnoptions', 'winver']

Слайд 48

Advanced Programming Spring 2002
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)

Слайд 49

Advanced Programming Spring 2002
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)

Слайд 50

Advanced Programming Spring 2002
Class definitions
Class ClassName:

...

must be executed
can be

executed conditionally (see Tcl)
creates new namespace

Слайд 51

Advanced Programming Spring 2002
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

Слайд 52

Advanced Programming Spring 2002
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

Слайд 53

Advanced Programming Spring 2002
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

Слайд 54

Advanced Programming Spring 2002
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

Слайд 55

Advanced Programming Spring 2002
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

Слайд 56

Advanced Programming Spring 2002
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)

Слайд 57

Advanced Programming Spring 2002
Notes on classes
Data attributes override method attributes with the

same name
no real hiding ? not usable to implement pure abstract data types
clients (users) of an object can add data attributes
first argument of method usually called self
'self' has no special meaning (cf. Java)

Слайд 58

Advanced Programming Spring 2002
Another example
bag.py
class Bag:
def init(self):
self.data = []
def

add(self, x):
self.data.append(x)
def addtwice(self,x):
self.add(x)
self.add(x)

Слайд 59

Advanced Programming Spring 2002
Another example, cont'd.
invoke:
>>> from bag import *
>>> l =

Bag()
>>> l.add('first')
>>> l.add('second')
>>> l.data
['first', 'second']

Слайд 60

Advanced Programming Spring 2002
Inheritance
class DerivedClassName(BaseClassName)

...

search class attribute, descending chain

of base classes
may override methods in the base class
call directly via BaseClassName.method

Слайд 61

Advanced Programming Spring 2002
Multiple inheritance
class DerivedClass(Base1,Base2,Base3):

depth-first, left-to-right
problem: class derived from two

classes with a common base class

Слайд 62

Advanced Programming Spring 2002
Private variables
No real support, but textual replacement (name mangling)
__var

is replaced by _classname_var
prevents only accidental modification, not true protection

Слайд 63

Advanced Programming Spring 2002
~ C structs
Empty class definition:
class Employee:
pass
john = Employee()
john.name =

'John Doe'
john.dept = 'CS'
john.salary = 1000

Слайд 64

Advanced Programming Spring 2002
Exceptions
syntax (parsing) errors
while 1 print 'Hello World'
File "",

line 1
while 1 print 'Hello World'
^
SyntaxError: invalid syntax
exceptions
run-time errors
e.g., ZeroDivisionError, NameError, TypeError

Слайд 65

Advanced Programming Spring 2002
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

Слайд 66

Advanced Programming Spring 2002
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

python

Содержание

Advanced Programming Spring 2002IntroductionMost recent popular (scripting/extension) languagealthough origin ~1991heritage: teaching language

Advanced Programming Spring 2002Python philosophyCoherencenot hard to read, write and maintainpowerscoperapid development

Advanced Programming Spring 2002Python featuresLutz, Programming Python

Advanced Programming Spring 2002Python featuresLutz, Programming Python

Advanced Programming Spring 2002Pythonelements from C++, Modula-3 (modules), ABC, Icon (slicing)same family

Advanced Programming Spring 2002Uses of Pythonshell toolssystem admin tools, command line programsextension-language

Advanced Programming Spring 2002What not to use Python (and kin) formost scripting

Advanced Programming Spring 2002Using python/usr/local/bin/python#! /usr/bin/env pythoninteractive use Python 1.6 (#1, Sep 24

Advanced Programming Spring 2002Python structuremodules: Python source files or C extensionsimport, top-level

Advanced Programming Spring 2002First example#!/usr/local/bin/python# import systems moduleimport sys marker = '::::::'for

Advanced Programming Spring 2002Basic operationsAssignment:size = 40a = b = c =

Advanced Programming Spring 2002String operationsconcatenate with + or neighborsword = 'Help' +

Advanced Programming Spring 2002Listslists can be heterogeneousa = ['spam', 'eggs', 100, 1234,

Advanced Programming Spring 2002Basic programminga,b = 0, 1# non-zero = truewhile b

Advanced Programming Spring 2002Control flow: ifx = int(raw_input("Please enter #:"))if x <

Advanced Programming Spring 2002Control flow: fora = ['cat', 'window', 'defenestrate']for x in

Advanced Programming Spring 2002Loops: break, continue, elsebreak and continue like Celse after

Advanced Programming Spring 2002Do nothingpass does nothingsyntactic filler while 1: pass

Advanced Programming Spring 2002Defining functionsdef fib(n): """Print a Fibonacci series up to

Advanced Programming Spring 2002Functions: default argument valuesdef ask_ok(prompt, retries=4, complaint='Yes or no,

Advanced Programming Spring 2002Keyword argumentslast arguments can be given as keywordsdef parrot(voltage,

Advanced Programming Spring 2002Lambda formsanonymous functionsmay not work in older versionsdef make_incrementor(n):

Advanced Programming Spring 2002List methodsappend(x)extend(L)append all items in list (like Tcl lappend)insert(i,x)remove(x)pop([i]),

Advanced Programming Spring 2002List methodscount(x)how many times x appears in listsort()sort items

Advanced Programming Spring 2002Functional programming toolsfilter(function, sequence)def f(x): return x%2 != 0

Advanced Programming Spring 2002List comprehensions (2.0)Create lists without map(), filter(), lambda= expression

Advanced Programming Spring 2002List comprehensionscross products:>>> vec1 = [2,4,6]>>> vec2 = [4,3,-9]>>>

Advanced Programming Spring 2002List comprehensionscan also use if:>>> [3*x for x in

Advanced Programming Spring 2002del – removing list itemsremove by index, not valueremove

Advanced Programming Spring 2002Tuples and sequenceslists, strings, tuples: examples of sequence typetuple

Advanced Programming Spring 2002TuplesTuples may be nested>>> u = t, (1,2)>>> u((123,

Advanced Programming Spring 2002TuplesEmpty tuples: ()>>> empty = ()>>> len(empty)0one item ?

Advanced Programming Spring 2002Tuplessequence unpacking ? distribute elements across variables>>> t =

Advanced Programming Spring 2002Dictionarieslike Tcl or awk associative arraysindexed by keyskeys are

Advanced Programming Spring 2002Dictionariesno particular orderdelete elements with del>>> del tel['foo']keys() method

Advanced Programming Spring 2002Conditionscan check for sequence membership with is and is

Advanced Programming Spring 2002ConditionsCan assign comparison to variable:>>> s1,s2,s3='', 'foo', 'bar'>>> non_null

Advanced Programming Spring 2002Comparing sequencesunlike C, can compare sequences (lists, tuples, ...)lexicographical

Advanced Programming Spring 2002Comparing sequences(1,2,3) < (1,2,4)[1,2,3] < [1,2,4]'ABC' < 'C' <

Advanced Programming Spring 2002Modulescollection of functions and variables, typically in scriptsdefinitions can

Advanced Programming Spring 2002Modulesimport module:import fiboUse modules via "name space":>>> fibo.fib(1000)>>> fibo.__name__'fibo'can

Advanced Programming Spring 2002Modulesfunction definition + executable statementsexecuted only when module is

Advanced Programming Spring 2002Module search pathcurrent directorylist of directories specified in PYTHONPATH

Advanced Programming Spring 2002Compiled Python filesinclude byte-compiled version of module if there

Advanced Programming Spring 2002Standard modulessystem-dependent listalways sys module>>> import sys>>> sys.p1'>>> '>>>

Advanced Programming Spring 2002Module listinguse dir() for each module>>> dir(fibo)['___name___', 'fib', 'fib2']>>>

Advanced Programming Spring 2002Classesmixture of C++ and Modula-3multiple base classesderived class can

Advanced Programming Spring 2002Classesclasses (and data types) are objectsbuilt-in types cannot be

Advanced Programming Spring 2002Class definitionsClass ClassName: ... must be executedcan be

Advanced Programming Spring 2002Namespacesmapping from name to object:built-in names (abs())global names in

Advanced Programming Spring 2002Namespacesscope = textual region of Python program where a

Advanced Programming Spring 2002Class objectsobj.name references (plus module!):class MyClass: "A simple example

Advanced Programming Spring 2002Class objectsclass instantiation:>>> x = MyClass()>>> x.f()'hello world'creates new

Advanced Programming Spring 2002Instance objectsattribute referencesdata attributes (C++/Java data members)created dynamicallyx.counter =

Advanced Programming Spring 2002Method objectsCalled immediately:x.f()can be referenced:xf = x.fwhile 1: print

Advanced Programming Spring 2002Notes on classesData attributes override method attributes with the

Advanced Programming Spring 2002Another examplebag.pyclass Bag: def __init__(self): self.data = [] def

Advanced Programming Spring 2002Another example, cont'd.invoke:>>> from bag import *>>> l =

Advanced Programming Spring 2002Inheritanceclass DerivedClassName(BaseClassName) ... search class attribute, descending chain

Advanced Programming Spring 2002Multiple inheritanceclass DerivedClass(Base1,Base2,Base3): depth-first, left-to-rightproblem: class derived from two

Advanced Programming Spring 2002Private variablesNo real support, but textual replacement (name mangling)__var

Advanced Programming Spring 2002~ C structsEmpty class definition:class Employee: passjohn = Employee()john.name =

Advanced Programming Spring 2002Exceptionssyntax (parsing) errors while 1 print 'Hello World' File "",

Advanced Programming Spring 2002Handling exceptionswhile 1: try: x = int(raw_input("Please enter a

Advanced Programming Spring 2002Handling exceptionstry.py import sys for arg in sys.argv[1:]: try: f =

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try:
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try.py
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