Python

Generic features and syntax

  • class.__mro__ = Contains all the ancestors of the class, up till object, the Python base class
  • _ = At Python prompt, it represents the last value. Mostly like $? in Bash
  • __file__ = Contains the name of the file loaded (a module if importing or the filename of the script)
  • locals() = Returns a dictionary with the local namespace
  • globals() = Returns a dictionary with the global module namespace. Inside a class or function returns the module where this is declared, not where it’s being executed
  • enumerate() = Returns a list’s elements with numeric indexes. For example, 0, abc, 1, def etc.
  • [::2] = Slice of a list, iterating every 2 elements. Works also by selecting a range ([1:10:2])
  • [::-1] = Iterate over a list backward
  • print("The {foo} is a {bar}".format(foo='answer', bar='42')) = String substitution in print using an alphanumerical index
  • first,*rest,last = (1,2,3,4,5,6,7) = Assign first element of the tuple to first, the last to last, and all those in between to rest. Python 3 only
  • first,second,*rest = 1,2,3,4,5,6 = Similarly to above, assign 1 to first, 2 to second and from 3 onward to rest. Rest will result in a list ([3,4,5,6]). Python 3 only
  • __qualname__ = Attribute of classes and functions showing the “path” from the module top-level to their definition. Practically shows all the parents functions/classes of a function/class. Only in Python >=3.3
  • python -m pydoc -k keyword = Search Python documentation for “keyword”
  • python -m this = The Zen of Python ;)
  • a[-3] = Negative indexing, returns the third to last element of a list
  • lastthree = slice(-3, None) = Saving a slice in a variable

Decorators

Write a function like this:

def func1(func):
	   def wrapper():
			# code to execute before func()
			func()
			# code to execute after func()
	   return wrapper

and then pass to a function this way:

@func1
def func():
	# function code

Metaclasses

Syntax:

ClassName = type(
	   'ClassName',
	   (object, ),
	   {'abc': def, 'esse': erre}
)

where ClassName is the name of the metaclass, (object,) is a tuple with all the classes to inherit and the following dictionary is the namespace of the metaclass. To refer to an instance of a metaclasse, use cls instead of self

Then to instantiate a metaclass:

MyClass = ClassName('MyClass', (object, ), {})

or it’s possible to instantiate the metaclass directly when creating a class, like:

class MyClass(object):
	  __metaclass__ = MyMetaClass

Set operations

various operations possible over sets. Examples:

A = {1, 2, 3, 3,}
B = {3, 4, 5, 6, 7}
  • A | B -> set([1, 2, 3, 4, 5, 6, 7])
  • A & B -> set([3])
  • A - B -> set([1, 2])
  • B - A -> set([4, 5, 6, 7])
  • A ^ B -> set([1, 2, 4, 5, 6, 7])

Modules

Bisect

  • bisect.insort(l, e) = Insert e in list l, automatically sorting the list. For huge lists it should be quicker than using .sorted()

Calendar

  • class HTMLCalendar = Format to html a calendar. After instantiating, use it like i.formatmonth(2012,11) to obtain the html code for november 2012
  • python -m calendar 2013 = Outputs the calendar for the 2013. It can be customized to print in html or text format, with or without a css and by width, lines, spacing etc. (see python -m calendar -h)

Cgitb

  • cgitb.enable() = Enable a very verbose traceback. With argument format="html" the output is html

Collections

  • class Counter = A class to count the occurrences of a character or integer inside a list, dictionary or string. Returns a dictionary like {'a': 3, 'b': 2, 'c': 2}
  • collections.deque = A different kind of queue, where values can be pushed or popped from both sides and not only one like in normal queues
  • collections.OrderedDict = A dictionary that remembers the order in which values have been inserted and when iterated upon, returns value in that exact order. Consequently, 2 OrderedDict with the same values but in a different order will not be considered equals, whereas a 2 normal dictionaries would

Named tuples

Point = collections.namedtuple('Point', ['x', 'y'])
p = Point(x=1.0, y=2.0)

Dis

  • dis.dis(code_object) = Disassemble un code_object

Itertools

Permutations

for p in itertools.permutations([1, 2, 3, 4]):
	print(''.join(str(x) for x in p))

prints all the possible combinations

1234
1243
1324
1342

etc.

Json

  • python -m json.tool < file.json = Pretty JSON printer

Os.Path

  • os.path.sep = Variable containing the path separator of the current OS (’' or ‘/’)
  • os.path.extsep = Variable containing the filename/extension separator (usually, ‘.’)
  • os.path.curdir = Variable containing the symbol of the current directory (’.’ in Unix-like sytems)
  • os.path.pardir = Variable containing the representation of the parent directory (’..’ in Unix-like systems)

Platform

Module to obtain various informations on the system. For example:

  • platform.system() = The OS name (like “Linux” or “Windows”)
  • platform.node() = The hostname
  • platform.release() = The release number of the OS (on Linux is usually the kernel version)
  • platform.machine() = The kind of hardware of the machine (like ‘i386’)
  • platform.architecture() = The architecture of the machine (nearly always nowadays, 32 or 64bit). Giving a file as argument, returns the architecture of it instead of the machine

Socket

  • socket.gethostname() = Returns the hostname
  • socket.gethostbyname(host) = Returns the IP of host
  • socket.gethostbyname_ex(host) = Returns all the IPs and aliases that belong to host

String

  • string.capwords(s) = Capitalize string s

Sys

  • sys.getrefcount(object) = Returns the number of times object is used. Includes a spurious copy created by getrefcount itself. Object can be a variable, a class or a function
  • sys.getsizeof(object) = Returns how much memory is occupying object. For classes, doesn’t include also all the attributes
  • sys.modules = Contains the currently loaded modules (recursively)

Time

  • time.clock() = Returns the cpu time, the number of clocks from the execution of the current program. Useful to measure performances because clock() doesn’t increases during idle time but only when actually running

Timeit

  • python -m timeit '".".join(str(n) for n in range(100))' = Quick measuring of speed of code snippets