python_notes.md

Week 3 (self study) 9-15 May 2016

---

source

Why Python is a great first language

• great experience from day 1 (clear code)
• can be used for web programming
• can be used for desktop programming
• marketable professional skill, easier to learn other languages after it
• good community (supportive, welcoming)

Style guide

• CamelCase for classes and lower_case_with_underscores for functions and method
• put comments on a line of their own
• Wrap lines so that they don’t exceed 79 characters
• Use 4-space indentation, and no tabs

Running python

• quit python interpreter: ^D or quit()
• chmod +x my_first_simple_script.py
• chmod 755 scriptname

Variables

• variable: poiting to an object (the value of the variable)
• object: data (built in/from extension library/created in the program)
• id(variable): the unique ID number of the object the variable is ponting to
• variable/identifier name can be [a-zA-z0-9_], but first char can not be [0-9]
• keywords:

  and, as, assert, break, class, continue, def, del, elif, else, except, False, finally, for, from, global, if, import, in, is, lambda, None, nonlocal, not, or, pass, raise, return, True, try, while, with, yield

Data types

• numerics
  • integer
    • normal
    • octal
    • hexadecimal
    • binary
  • floating point
  • complex
• sequences
  • strings
  • byte sequences
  • byte arrays
  • lists
  • tuples
  • range objects
• mappings
• files
• instances
• exceptions

Numbers

• int(), float(), complex() to convert between types
• integer: unlimited size, type(x), isinstance(x, complex) normal
  • octal: prefix 0o or 0O
  • hexadecimal: prefix 0x or 0X
  • binary: prefix 0b or 0B
  • hex(), oct(), bin(), int() to convert (to string)
• long integer: only used in Python2
• floating-point: for example: 42.11, 3.1415e-10, accurate only up to 15 decimals
• complex: <real part> + <imaginary part>j
• integer division 10/3 is floor division in Python2 but true division in Python3 where 10//3 is floor division (truncating fractional digits)

Strings

• s = 'It doesn\'t matter!' OR "He said: \"It doesn't matter\""
• s = '''multiple line string''' OR s = """multiple line string"""
• s[0] first char of string, s[len(s)-1] OR s[-1] last char of string,
• concatenation +, repetion *, indexing s[0], slicing s[2:4], size len(s)
• immutable, not possible to replace individual characters
• a is b -> True if they point to the same object ID (if assigned separately they get the same ID if hyphen is not part of the string)
• escape characters with \
• string objects: stored as a sequence of characters

Byte strings

• stored as a sequence of bytes (just as date objects)
• x = b"Hello"
• s = str(x) -> convert to string

Lists

x = [], x = [1, 1.0, "hello", [1, 2, 3]] empty, mixed types, nested x[2][1] nested indexing, from the above list it yields "e"

• negative indexing: counting from the end backwards
• slicing: x[begin: end: step]
• mutable x[0] = 3
• add 1 item: x.append(7), add more items: x.extend([5, 6, 7])
• concatenation +, repetition * creates more references to the same list and concatenates them!
• insert 1 element to position [1]: x.insert(1, "hello")
• squeeze more elements to position starting at [2]: x[2:2] = ["szia", "mia"]
• del x[2], del x[2:4], del x to delete the entire list
• x.remove("hello") first occurrence, x.pop(3) return and delete x[3], x.pop()return and delete last elementx[-1]`
• x.clear() empty the list. x[2:5] = []
• x.index("hello") what is the index of first occurrence of "hello" in x
• x.count("hello") how many occurrences
• sort(), reverse() ascending order, reverse order
• copy() return a shallow copy of the list

Tuples

• t = (1, 2, 3) define with parentheses
• immutable, can be used as keys in dictionaries

List methods

Flow control

while <condition>:
    <statement>
else:
    <statement>

for <variable> in <sequence>:
    <statements>
else:
    <statements>

range(<begin>, <end>, <step>)

• if statement
• colon, indentation
• false: 0, None, False, true: non-zero value, True
• if...elif...elif...elif...else

#!/usr/bin/env python3

gender = input("Gender? ")
if gender == "male" or gender == "Male":
    print("Your cat is male")
else:
    print("Your cat is female")

age = int(input("Age of your cat? "))
if age < 5:
    print("Your cat is young.")
else:
    print("Your cat is adult.")

• for loop
• while loop
• break and continue
• pass

Operators:

• ==, <=, >=, !=, <, >
• and, or, not
• ~x, |, &, ^: bitewise negation, OR, AND, XOR
• <<, >>: shift operators
• in: "element of"
• ** exponential, % modulo, // truncation division, +, -, *, /

Functions

def function_name(parameter1, optional_parameter="defaul value", *arbitrary_number_of_values):
    """the text of the docstring"""
    statements
    global x # make variable defined inside the function global
    return # optional, gives None if not specified
print (function_name.__doc__)

call_function(*x) to unpack the list x

Files

f = open('workfile', 'w')

• 'r': read only (default), 'w': write only, 'a': append, 'r+': read-write
• append 'b' to above options to open as binary for all non-text files
• f.read(<size>), f.read(), f.readline(), f.readlines() list of lines
• f.write(string). f.writelines("a", "b", "another line")
• f.close()

with open('workfile', 'r') as f:
    read_data = f.read()
f.closed # this tests if f has been closed (should return True)

f.mode, f.name

Code examples

print(value1, ..., sep=' ', end='\n', file=sys.stdout, flush=False) input("prompt: ") returns a string (can be used with eval(), int() etc.)

#!/usr/bin/env python3
#!/usr/local/bin/python3.5
# -*- coding: encoding -*-

print("Hello, World!")

"""
this is a multiline comment
"""

if gender == "male" or gender == "Male":
    command
else:
    other command
command outside of condition