====== Python Cheat Sheet ======

[[http://www.python.org/ | Python]] is a dynamic object-oriented programming language with extensive standard libraries. These notes summarize the [[http://docs.python.org/3.1/tutorial/ | Python 3.1 tutorial]].
  * **python**/**python3.1** is the command-line interpreter with readline where available.
  * python -c command runs a single command.
  * %%>>> prompts for a line . ... prompts for a continuation of a previous line.%%
  * PYTHONSTARTUP can point to a file of startup commands
  * Expressions are evaluated and printed.
  * The last printed expression is assigned to _ .

===== Basics =====
  * Comments start with # .
  * sys.argv is a list of strings with the script if any in sys.argv[0]
  * Variables can be assigned (using = ) without being defined but must be assigned before being used.
  * a,b = c,d does a multiple assignment.
  * Imaginary numbers can be created using complex(a,b) or a+bJ. .real and .imag will extract real and imaginary parts.
  * print("a","b",end="") prints a b with no newline at the end.

===== Functions =====
  * To define a function or procedure <code python>define func(n,str="default"):</code> <code python>define func(formal1, formal2, *arguments **keywords)</code>
  * The default value for an argument is evaluated only once when the function is defined.
  * All arguments are passed using //call by object reference//. So changes are seen by caller.
  * * unpacks a list or dictionary for use in a function.
  * lambda x: x + n creates a small anonymous function.
  * A function should contain a triply quoted doc string at the start. This is accessible with func.__doc__ .
  * 4 spaces per indent, no tabs. CamelCase for classes lower_case with underscores for functions.

===== Lists =====
  * list=[1 ,'a' , "b"] is a list of three items.
  * list[4] is 4th element. list[2:4] is second and third elements. list[:2] is first 2 elements. list[2:] is third element to last one. list[:] is the whole list.
  * Negative indexes count from last element.
  * List slices can be modified. list[3:3] can be used to insert elements at position 3.
  * len(list) returns length of the list.
  * Lists can be nested.
  * Other methods include append, pop, extend, index, count, sort, reverse.
  * Lists can be created from sequences using list comprehension <code python>
vec = [2, 4, 6]
[[x, x**2] for x in vec if x > 2]
[[4, 16], [6, 36]]
[x+y for x in vec for y in vec2 if x > 2]</code>
  * Other sequence types include tuples ( (1,2) or 1,2) and sets ({1,2} or set(1,2)).

===== Input/Output =====
  * Strings can be enclosed with double or single-quotes.
  * \ in a string by itself goes to the next line but does not insert a newline (\n does).
  * A raw string - r"" does no escaping.
  * + concatenates strings, * repeats them .
  * A string can be treated as a list of letters.
  * But strings can't be modified using slices or indexes.
  * [[http://docs.python.org/3.1/library/stdtypes.html#string-methods | String methods]] include strip, format, capitalize etc.
  * //formatstring.format(tuple)// is the [[http://docs.python.org/3.1/library/string.html#formatspec | new-style printf]] equivalent. The [[http://docs.python.org/3.1/library/stdtypes.html#old-string-formatting | old-style]] was //formatstring % (tuple)// which uses C-type format strings.
  * str() returns a readable representation, repr() returns a canonical representation of any object.
  * To open a file <code python> open('/tmp/workfile', 'w') </code>
  * [[http://docs.python.org/3.1/library/stdtypes.html#file-objects | File methods]] include read, readline, write.
  * //pickle// contains [[http://docs.python.org/3.1/library/pickle.html | load and dump methods]] for serializing objects. Support should be added for new object types.



===== Control Flow =====

  * Controlled by level of indentation.
  * //pass// is a NOOP .
  * <code python>
while a<10:
  pass</code>
  * <code python>
for x in list:
  pass
for x in range(1,5):
  pass</code>
  * <code python>
if a›10:
   pass
elif a›5:
   pass
else:</code>
  * //break// and //continue// can be used
  * Loops may have else clauses for code executed after normal termination (not a break).

===== Assorted Collection Stuff =====

  * Dictionaries are unordered sets of key value pairs e.g. { a:1, b:2} or dict([(a,1),(b,2)]) or dict(a=1,b=2)
  * Dictionary keys/values/items can be enumerated over.
  * sorted and reversed take in lists and return new lists.
  * enumerate takes in a list and returns a list of pairs where the first item in the pair is an integer starting with 0.
  * zip takes two or more lists with the same number of elements and creates a single list of lists with corresponding members grouped together.

===== Modules & Packages =====

  * A //module// is a file of python definitions.
  * Definitions in a module can be accessed by importing the module. <code python>import fib
from fib import fib1, fib2
</code>
  * %%__name__%% is set to the name of the module or to %%__main__%% if called from the command line.
  * The search list for modules is the variable sys.path initialized from the directory containing the input script (or the current directory), PYTHONPATH and the installation-dependent default.
  * When a module is imported, a byte-code version is stored to module.pyc which speeds up successive loads.
  * There are many standard modules including sys which is common to all platforms.
  * dir(module) lists all names in a module sorted alphabetically. dir(builtins) shows all the built-in functions and variables.
  * Packages can be used to group modules. Packages should be in a named directory with an __init__.py file optionally containing initialization code. E.g. import sound.effects.echo .
  * . is the current package, .. is the parent package etc.

===== Error Handling =====

  * Syntax errors are parsing errors, exceptions are runtime errors.
  * Exceptions are class objects in the exceptions module. [[http://docs.python.org/3.1/library/exceptions.html#bltin-exceptions | List of builtin exceptions]].
  * Exception handling is supported using try..except..else..finally :<code python>
try:
    result = x / y
except ZeroDivisionError:
    print("division by zero!")
except Exception as inst:
    print(type(inst))    # the exception instance
    print(inst.args)     # arguments stored in .args
    print(inst)          # also prints .args
else:
    print("result is", result)
finally:
    print("executing finally clause")
</code>
  * Errors can be raised using ''raise Exception('mine')'' or just ''raise'' in an exception handler.
  * New exception classes can be created and should derive from the Exception class.
  * ''with'' can be used to automatically call the cleanup actions of an object e.g. <code python>
with open("myfile.txt") as f:
    for line in f:
        print(line)
</code>

===== Classes =====