Transcript Introduction To Python

Sarah Farley
 High
level language
 Extensible
 No compilation or linking
 Type
python on the command line to start
the interpreter
 python -c command [arg] is an
alternative
 Exit by ctl- d or quit()
 >>> Is the prompt for next command
 … indicates a continuation
 Comments are indicated with #
 +, -, *, /
act as you would expect them to
 >>> (50-5*6)/4
• 5
 >>>
2+2
• 4
 >>>
• -3
7/-3
=
is used to assign variables
 >>> width = 20
• >>> height = 5*9
• >>> width * height
• 900
 Values
can be assigned simultaneously
 >>> x = y = z = 0
 When
working with mixed operators,
integers are turned into floats
 >>> 3 * 3.75 / 1.5
• 7.5
 >>>
• 3.5
7.0 / 2


Strings can be declared with ‘ or “.
>>> 'spam eggs'
• 'spam eggs'

>>> 'doesn\'t'
 "doesn't"

>>> "doesn't"
• "doesn't"

>>> '"Yes," he said.'
• '"Yes," he said.'

>>> "\"Yes,\" he said."
• '"Yes," he said.'

>>> '"Isn\'t," she said.'
• '"Isn\'t," she said.'
 To
input string across multiple lines use
the / character
 hello = "This is a rather long string
containing\n\ several lines of text just as
you would do in C.\n\ Note that
whitespace at the beginning of the line
is\ significant."
 Or “””
 print """ Usage: thingy [OPTIONS] -h
Display this usage message -H hostname
Hostname to connect to """
 To
get a raw string, you add an r in front
 >>>hello = r"This is a rather long string
containing\n\ several lines of text much
as you would do in C.“
 It prints the string exactly ignoring things
like newlines
 …This is a rather long string
containing\n\ several lines of text much
as you would do in C.
 Strings
are concatenated with + and
repeated with *
 >>> word = 'Help' + 'A'
 >>> word
• 'HelpA'
 >>> '<' + word*5 + '>'
• '<HelpAHelpAHelpAHelpAHelpA>'
 Placing two literal strings next
to each other
automatically concatenates them.
 >>> 'str' 'ing'
• 'string'


Strings are indexed, starting at 0
>>> word[4]
• 'A'

>>> word[0:2]
• 'He'

>>> word[2:4]
• 'lp'


Strings can also be sliced.
>>> word[:2]
• 'He'

>>> word[2:]
• 'lpA'

Giving negative numbers for the indexes starts the
string at the right instead of the left
 List
do not need to have the same data
type throughout
 >>> a = ['spam', 'eggs', 100, 1234]
 >>> a
• ['spam', 'eggs', 100, 1234]
 Like
strings, the index starts at 0 and lists
can be sliced, concatonated, etc.
It is possible to change a list once its been made.
>>> # Replace some items: ... a[0:2] = [1, 12]
>>> a [1, 12, 123, 1234]
>>> # Remove some: ... a[0:2] = []
>>> a [123, 1234]
>>> # Insert some: ... a[1:1] = ['bletch', 'xyzzy']
>>> a [123, 'bletch', 'xyzzy', 1234]
>>> # Insert (a copy of) itself at the beginning
>>> a[:0] = a
>>> a [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy',
1234]
 >>> # Clear the list: replace all items with an empty
list
 >>> a[:] = [] >>> a []
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 >>>
x = int(raw_input("Please enter an
integer: "))
 Please enter an integer: 42
 >>> if x < 0:
... x = 0
... print 'Negative changed to zero'
... elif x == 0:
... print 'Zero'
... elif x == 1:
... print 'Single'
... else:
... print 'More'
 >>>
# Measure some strings:
... a = ['cat', 'window', 'defenestrate']
>>> for x in a: ... print x, len(x)
... cat 3 window 6 defenestrate 12
 >>>
range(5, 10)
• [5, 6, 7, 8, 9]
 >>>
range(0, 10, 3)
• [0, 3, 6, 9]
 >>>
range(-10, -100, -30)
• [-10, -40, -70]
 The
break statement breaks out of the for
or while loop.
 The continue statement continues onto
the next section of the loop.
 The else clause terminates when the list
exhausts or becomes false.
 The pass funtion is a placeholder. It
doesn’t do anything.
 def()
is the default argument for a
function.
 def ask_ok(prompt, retries=4,
complaint='Yes or no, please!'):
 They
have the form keyword = value.
 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, "!"
 http://docs.python.org/tutorial/index.ht
ml