Python笔记

2017年5月25日 (四) 18:37的最后版本

[编辑] 1 Quick Start

Install python2.7 from https://www.python.org/downloads/

Install pip:

$ curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
$ sudo python get-pip.py

Using the pip to install the numpy and tensorflow package:

$ sudo pip install numpy
$ sudo pip install tensorflow

[编辑] 2 Basics

[编辑] 2.1 Numbers

Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
>>> a = 3.23
>>> b = 4
>>> c = 52.3E-4
>>> print a, b, c
3.23 4 0.00523

[编辑] 2.2 Strings

#!/usr/bin/python

str_1 = 'Jack Tan'
str_2 = "Ga Xiao Duo"
str_3 = '''This is a multi-line string. first line.
second line'''

# using the startswith method of the string object
if str_1.startswith('Ja'):
    print("Yes, str_1 start with 'Ja'")

if 'a' in str_1:
    print('Yes, str_1 contains the string "a"')

substr = 'ac'
if str_1.find(substr) != -1:
    print('Yes, str_1 contains the string "{0}"'.format(substr))
    print('Yes, {0} contains the string "{1}"'.format('str_1', substr))
    print('Yes, {} contains the string "{}"'.format('str_1', substr))
    # using the format method of the string object

deli = '_*_'
mylist = ['Brazil', 'Russia', 'India', 'China']

# using the join method of the string object
print(deli.join(mylist))

Output:

Yes, name start with 'Ja'
Yes, name contains the string "a"
Yes, name contains the string "ac"
Brazil_*_Russia_*_India_*_China

Once your have created a string, you cannot change it

[编辑] 2.3 Variable

Python refers to anything used in a program as an object.

Do not need type declaration of variable in python

Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
>>> a = 3.1415
>>> b = 100000
>>> c = 'Jack Tan'
>>> d = ['x1', 'x2']
>>> print a, b, c, d
3.1415 100000 Jack Tan ['x1', 'x2']

.

Variables can hold values of different types called data types. The basic types are numbers and strings. We can create our own types using classes.

[编辑] 2.4 Indentation

Whitespace at the beginning of the line is important!

Spaces and tabs at the beginning of the logical line are used to determine the indentation level of the logical line, which in turn is used to determine the grouping of statements.

Statements which go together must have the same indentation. Each usch set of statements is called a block

[编辑] 2.5 Format

#!/usr/bin/python

#width = 10,precise = 3,align = left
print "PI = %10.3f" %(math.pi)

# don't add the '\n' at the end of line
print "PI = %10.3f" %(math.pi),

#width = 10,precise = 3,align = rigth
print "PI = %-10.3f" % math.pi

Output:

PI = 3.141593
PI =      3.142

Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
>>> print "%.3s " % ("jcodeer") #precise = 3
jco 
>>> print "%.*s" % (4,"jcodeer") #precise = 4
jcod
>>> print "%10.3s" % ("jcodeer") #width = 10,precise = 3
       jco
>>>

fmat = "%r %r %r %r"
 
print fmat % (1, 2, 3, 4)
print fmat % ("one", "two", "three", "four")
print fmat % (True, False, False, True)

1 2 3 4
'one' 'two' 'three' 'four'
True False False True

print('{0:.3}'.format(0.3333333))
#print('{0:.3}'.format(1/3))
print('{0:_^11}'.format('Hello'))
print('{0:+^11}'.format('Hello'))
print('{name} wrote {book}'.format(name='Jack', book='Hello world'))

PI = 3.141593
PI =      3.142

0.333
___Hello___
+++Hello+++
Jack wrote Hello world

[编辑] 3 Operators

[编辑] 4 Control Flow

[编辑] 5 Functions

[编辑] 6 Data Structures

[编辑] 7 Exceptions

[编辑] 7.1 with...as

#!/usr/bin/python

import sys

class test:
    def __enter__(self):
        print("enter")
        return 1
    def __exit__(self, *args):
        print("clean up")
        return False
        #return True


with test() as t:
    print("t is not the result of test(), it's __enter__ returned")
    print("t is 1, yes, it's {0}".format(t))
    print "t is 1, yes, it's", t
    raise NameError("Error")

sys.exit()
print("Never here")