Python Cheat Sheet

Python is a dynamic object-oriented programming language with extensive standard libraries. These notes summarize the 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 _ .

• 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.

• To define a function or procedure

  define func(n,str="default"):

  define func(formal1, formal2, *arguments **keywords)

• 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.

• 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

  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]

• Other sequence types include tuples ( (1,2) or 1,2) and sets ({1,2} or set(1,2)).

• 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.
• String methods include strip, format, capitalize etc.
• formatstring.format(tuple) is the new-style printf equivalent. The 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

   open('/tmp/workfile', 'w') 

• File methods include read, readline, write.
• pickle contains load and dump methods for serializing objects. Support should be added for new object types.

• Controlled by level of indentation.
• pass is a NOOP .

• while a<10:
    pass

• for x in list:
    pass
  for x in range(1,5):
    pass

• if a›10:
     pass
  elif a›5:
     pass
  else:

• break and continue can be used
• Loops may have else clauses for code executed after normal termination (not a break).

• 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.

• A module is a file of python definitions.
• Definitions in a module can be accessed by importing the module.

  import fib
  from fib import fib1, fib2

• __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.

• Syntax errors are parsing errors, exceptions are runtime errors.
• Exceptions are class objects in the exceptions module. List of builtin exceptions.
• Exception handling is supported using try..except..else..finally :

  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")

• 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.

  with open("myfile.txt") as f:
      for line in f:
          print(line)