Python Cheat Sheet

Python is a dynamic object-oriented programming language with extensive standard libraries. These notes summarize the Python 3.0 tutorial.

• python/python3.0 is the command-line interpreter with readline where available.
• python -c command runs a single command.
• sys.argv is a list of strings with the script if any in sys.argv[0]
• »> 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 # .
• 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. CamelCasel 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 e.g

  vec = [2, 4, 6]
  [[x, x**2] for x in vec]
  [[2, 4], [4, 16], [6, 36]]

• 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.
• format-string % (tuple) allows use of printf formatting strings. Also string.format() .

• 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.