Jim Havrilla

Invoking Python Just type “python –m script.py [arg]” or “python –c command [arg]” To exit, quit() or Control-D is used To just use the interpreter just type “python” and just issue python commands (called interactive mode) FUN FACT: no semicolons needed in python! FUN FACT: in Python, the last printed expression can be referenced as an automatically stored variable called “_” in interactive mode

Argument passing To access arguments passed in a python script type “import sys” and the arguments are stored in sys.argv When no arguments are given, sys.argv[0] is an empty string

To handle error Type Control-C or DELETE (the interrupt character) cancels the input and returns to the main prompt If a command is being executed, it raises the KeyboardInterrupt exception which could be handled with a try statement

Things about Python scripts # designates comments “$ chmod +x myscript.py” to make a script executable in Unix, and “#! /usr/bin/env python” must go at the beginning of the script “import” at the beginning of a script to make it import things After the initial “#!” line one can define the source coding style with a special comment like “# -*- coding: iso *-” If you want the python interpreter to do certain things during start-up, you can set an environment variable called “PYTHONSTARTUP” to the name of the file containing the startup commands

Numbers in Python Python interpreter can be used as a straight up calculator like 2+2 will come out as 4 with no extra syntax Integer division/multiplication returns the floor, and integer + floating point operations return floating point An equal sign assigns a value to a variable but can do it to several variables at once like “x=y=z=0” Can also assign like so “a, b = 0, 1” and a=0, b=1 Variables don’t need type designation, but of course need a value to be used or else an error will occur Complex numbers are supported as well in the form of a = complex(real,imaginary) or a = j and returned as (real+imaginaryj); can be accessed in parts like a.real, a.imag, or abs(a) gives its magnitude

Strings in Python Strings can be in single or double quotes, but usually in single quotes unless the string is a single quote in two double quotes A string can span several lines of code; newline character is \n, and to indicate that the next line is a continuation of the last the \ character is used Type “print” to print a string or just a given value of a whole expression Can also surround strings in triple quote pairs “”” “”” or ‘’’ ‘’’ and then newline characters don’t need escaping

More about strings in Python Strings can be concatenated with “+” and if the strings are just string literals they can be concatenated with just a space between them like word = “lame” “Ya” word = “lameYa” Strings can be indexed like char arrays in C, so word[0] = l Colon has different effects in indexing: “word[0:2] = la” or “word[:2] = la” or “word[2:] = meYa” If an index is too large it is replaced by the string size like for example “word[1:1000] = ameYa” If an index is too small or too negative it is truncated unless it is a single-element reference (yes, you can use negative indices as long as the last element is 0)

More on strings Can’t change parts of strings in python, but can easily create a new string len(string) returns its length, but length of a slice is index2-index1 u‘string’ indicates a unicode based string and Python Unicode-Escape encoding can be used for characters then or ur‘string’ for raw unicode unicode() gives access to all Unicode codecs, unicode strings can be converted with str() to normal strings string.encode(‘utf-8’) will encode in 8-bit or in whatever coding is specified in encode()

Lists in Python list = [123, ‘abc’] is a list Can be indexed and concatenated like strings (but not using a space) Slice operations return a new list of the indexed elements Individual elements of a list can be changed and can be replaced by slice operations like list[0:1] = 3  list = [3, ‘abc’] len() still gives length, range() generates arithmetic progressions of numbers in a list like range(5,20,5) = [5,10,15,20] lists can contain other lists mixed with single elements like list = [[‘k’, 23], ‘f’, 5]

Some syntax There is no {} for while or for loops or for if statements, there must be a tab or spaces for each indented line, everything within a block by the same amount Loop conditions don’t need to be written in parentheses To instead print spaced answers instead of by newline with print in a while loop use print b, next line instead of print b next line (trailing comma dodges the newline after output)

Control Flow in Python If statements don’t need elif or else parts but can have as many elifs as needed, “if elif elif” statement is equivalent to a switch or case statement For statements work as an iteration over items of a list or sequence like “for x in list[:]:” and then the items are executed in order that they appear Can use “for i in range(len(list)): print i, a[i];” to get an iteration through the list easily

More on Control Flow break statement breaks out of smallest enclosing loop, to move on with going to “else” in an if statement for example continue statement moves on with next iteration of the loop pass statement does nothing and is used syntactically for things like empty classes or used as a placeholder when you are working on a function or condition

Functions in Python The term “def” introduces the definition of a function and is followed by a function name and in parentheses a list of arguments def statement is like “def func(number):” By default all variable assignments in a function store local variables return is used to give back a result Function can be renamed as a variable and stored to be used in the interpreter like “f = func f(423)” >>> output variable.append(new stuff) is the more efficient way to concatenate new elements at the end of a list in a function

Some more function stuff To assign default values one just needs to set each argument in the definition to a value like: “def func(number=100, why=“not”, tiny)” Also, “in” can be used to test if a sequence has a value like “if func in (‘no’, ‘not’) return True” If the default value is a mutable object, it is evaluated only once, and a function can accumulate arguments like “def func(thing, list=[]) list.append(thing) return list” for “print func(1), func(‘f’), func(10)” This would print “[1], [1,’f’], [1,’f’,10]” To fix the above just put if list is None for each call so that it doesn’t append continually

A bit more on functions If a function is written like so “def func(arg, *name, **args)” then the formal parameter **args acts as dictionary containing the keyword arguments except for the whats already a formal parameter or *name in this case which just contains the positional arguments (a variable number not including ** args after *name) Like in def func(arg1, *name, **args) for arg in name: print arg key = args.keys() for keyword in key: print keyword, “-”, key[keyword] “func(“hi”, “bye”, “try”, haha = “three”, nine = “five”)” would make bye try haha – three nine – five Functions can be called with arbitrary number of arguments Dictionaries can deliver keyword arguments into a function using the ** operator and the *operator can unpack arguments from a list/tuple lambda function can be used to make small anonymous function inside function definition Like “def super(arg): return lambda q: q * arg” can be used to say “func=super(5) func(5)” func(5) is equal to 25 and func(10) = 50

Documentation and Style Try using 4-space indentation instead of tabs Type a string in a function definition without print in front of it and it can be seen by type func.__doc__ to see what is called the docstring

Done! Any questions?