卖萌的弱渣

I am stupid, I am hungry.

Python: Introduction

Study Resource

Features

  • Advancced Data Structure
  • Rely on indentation (- -!)
  • No declaration for parameters

Interpreter

  • Boot with a script
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python test.py
  • Boot with command
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python -c command [arg]
  • Boot with module
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python -m module [arg]

Module name and command parameters are stored in sys.argv[]

  • Make your python script executable

    • Add the following in the first line
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#! /usr/bin/env python3.4
  • Give your file execution permission
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chmod +x yourpython.py
  • Use special character encoding Add the following in the first or second line for Windows 1252
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# -*- coding: cp-1252 -*-

Basic Introduction

Number

  • Division will not dismiss fractional part
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>>> 8/5
1.6
  • If you do not like fractional part, use //.
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>>> 7//3
2
>> 7//-3
-3
  • Python will transfer int to float if your expression has both of them.
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>>> 3 * 3.75 / 1.5
7.5
>>> 7.0/2
3.5

Assignment

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>>> width = 20
>>> x = y = z = 10          # Assign multiple var with same value
>>> x
10
>>> y
10
  • The last printed expression is assigned to the variable _
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>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

> `_` variable is read-only by the user.
  • You can also do the multiple assignment
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a,b = 0,1   # a=0, b=1

Parameter

Must define the parameter before you access it

For example:

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>>> n
NameError: xxxxxxxxx

String

String can be expressed in ‘ ’ or “ ”

  • If you use “ ‘ ” or ’ “ ‘, the inside quote will be considered quote. Otherwise, you need use \
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>>> 'eggs'
'spam eggs'
>>> 'doesn\'t'     # use \' to represent single quote because this is ' ' '
"doesn't:
>>> '"Yes," he said.'  # you don't need \" because it's ' " '
'"Yes," he said.'
>>> "\"Yes,\" he said."  # you have to use \" for ", because this is " " "
'"Yes," he said.'
>>> '"Isn\'t," she said,'
'"Isn\'t," she said.'
  • print() will handle ‘\’ as special character
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>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
>>> print('"Isn'\t,"she said.')
"Isn't," she said.         # \ will not be displayed
>>> s = 'First line, \nSecond line.'
'First line, \nSecond line.'
>>> print(s)
First line,
Second line.

If you want python accpet \, you need to add r before the first quote

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>> print('C:\some\name')
C:\some
ame
>>> print(r'C:\some\name')
C:\some\name
  • print() can display multiple line by using “ ” ”
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print("""\
Usage: thingy
       -h
      -H
""")

Output is the same format as this input

  • String can be concatenated with + and repeated with *
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>>> 3 * 'un' + 'iun'
'unununiun'
  • String literals next to each other are automatically concatenated
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>>> 'Py' 'thon'
'Python'

This automatical concatenation Cannot concatenate a variable and a string

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>>> prefix = 'Py'
>>> prefix 'thon'
SyntaxError: invalid syntax

But + still works

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>>> prefix ='Py'
>>> prefix + 'thon'
'Python'
  • String can be indexed starting from 0. string[0] is also string type
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>>> word = "Python"
>>> word[0]
'P'
>>> word[5]
'n'
  • Indices may also be negative numbers which start counting from the right, negative indices starts from -1
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>>> word[-1]
'n'
>>> word[-2]
'o'
>>> word[-6]
'P'
  • Slicing is supported in string. We can get substring by slicing
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>>> word='Python'
>>> word[0:2]    # get characters [0,2)
'Py'
>>> word[:2]
'Py'
>>> word[4:]    # get characters [4,length]
'on'
>>> word[-2:]
'on'
>>> word[:2] + word[2:]
'Python'
  • Attempting to use a index that is too large will result in error
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>>> word='Python'
>>> word[42]
IndexError: string index out of range

But slice index is ok for this

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>>> word='Python'
>>> word[4:42]
'on'
>>> word[42:]
' '
  • Python strings cannot be changed
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>>> word[0] = 'J'
TypeError: 'str' does not support item assign

If you need a different string, you have to create a new one

  • Function len() returns the length of a string
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>>> s = 'abcdefg'
>>> len(s)
7

Lists

  • Lists can contain items of different types, but usually the items have the same type
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>>> squares = [1,4,9,16,25]
>>> squares
[1,4,9,16,25]
  • Lists can also be indexed and sliced:
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>>> squares = [1,4,9,16,25]
>>> squares[0]
1
>>> square[-1]
25
>>> squares[-3:]            // slice will return a new list
[9,16,25]
  • List can also be concatenated
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>>> squares = [1,4,9,16,25]
>>> squares + [36,49,64,81,100]
[1,4,9,16,25,36,49,64,81,100]
  • Strings cannot be changed, but list can be changed
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>>> cubes = [1,8,27,65,125]
>>> cubes[3] = 64
>>> cubes
[1,8,27,64,125]
  • You can add new items at the ned of list by append()
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>>> cubes = [1,8,27,64,125]
>>> cubes.append(216)
>>> cubes.append(7**3)
>>> cubes
[1,8,27,64,125,216,343]
  • You can even change the size of list or clear it by assignment to slices
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>>> letters = ['a','b','c','d','e','f','g']
>>> letters[2:5] = ['C','D','E']
>>> letters
['a','b','C','D','E','f','g']
# remove them
>>> letters[2:5] = []
>>> letters
['a','b','f','g']
>>> letters[:] = []
>>> letters
[]
  • len() can applies to lists
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>>> letters = ['a','b','c','d']
>>> len(letters)
4
  • A lists can contain other lists
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>>> a = ['a','b','c']
>>> n = [1,2,3,]
>>> x= [a,n]
>>> x
[['a','b','c'],[1,2,3]]
>>> x[0]
['a','b','c']
>>> x[0][1]                 # like two dimention array
'b'