Python Quick review

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Python Quick review

• CS360
• Dick Steflik

2
Starting with Hello World

• start the python shell type print
  "Hello World!"
  orput into file hello.py with text editor
  restart the python shell and type
  import hello
  orat OS command prompt type python
  hello.py

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Variables, Expressions and Statements

• type(ltexpressiongt) returns the type of the
  expression print ltexprgt, ..., ltexprgt print
  several expressions ltvariablegt ltexpressiongt
  assign value to variable operators - /
  normal arithmetic operators operators
  is exponentiation, is mod () gt gt / gt
  - precedence (otherwise mostly
  
  left-to-right) ltvargt raw_input(ltpromptgt) read
  a text line of input ltvargt input(ltpromptgt)
  read a numerical line of input text
  comment

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Functions

• int(ltfloat or stringgt) convert to int
  str(ltnonstring expressiongt) convert to string
  import math make math functions
  availmath.sin, math.cos, math.sqrt examples of
  math functions help('math') list of all math
  functions
  (q to quit help) defining
  functions def ltnamegt(ltparamgt, ...,
  ltparamgt) ltstatementsgtreturn statement
  (immediately exits function, returning
  a value) return ltexpressiongt

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Conditionals

• type boolean (logical tests) either True or
  False x ! y x is not equal to y x gt y
  x is greater than y x lt y x is less
  than y x gt y x is greater than or equal to
  y x lt y x is less than or equal to y x
  and y x and y are both true x or y x or
  y (or both) are true not x x is not true

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if/else statements

• if lttestgt ltstatementsgt if lttestgt
  ltstatementsgt else ltstatementsgt if
  lttest1gt ltstatementsgt elif lttest2gt
  ltstatementsgt else ltstatementsgt

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Strings

• strings are immutable to change one construct a
  new onesindex return character at index (0
  based) len(s) length of string s-1, s-2,
  s-3 last character, 2nd to last, 3rd to last
  sij slice of string i through (j - 1)
  sj slice from beginning through (j - 1)
  si slice from i to end of string ltstringgt
  ltstringgt concatenate two strings help('str')
  show string functions (q to quit help)
  s.lower() returns lowercase version of s
  str.isalpha is alphabetic character (boolean
  function) foreach loop over string for
  ltvariablegt in ltstringgt ltstatementsgt

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Lists

• ltexprgt, ..., ltexprgt a list same use of , len
  lists are very similar to stringsrange(n)
  list of integers 0 through (n - 1)range(i, j)
  list of integers i through (j - 1)
  empty list ltexpressiongt in ltlistgt test for
  list membership (boolean) ltlistgt ltlistgt
  append two lists together to form new list
  list1 list(list2) make a copy of list2,
  store in list1 ltvargt list(ltstringgt) convert
  string to list ltlist vargt.append(ltexprgt) append
  value to end of list ltlist vargt.remove(ltexprgt)
  remove given value from list del lsti
  remove value at index i foreach loop over
  list for ltvariablegt in ltlistgt ltstatementsgt

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Files

• ltvargt open(ltnamegt, 'r') open a file for input
  ltvargt open(ltnamegt) same as above, r is
  default ltfile variablegt.readLine() return next
  line of file as a string ltfile
  variablegt.readLines() return all lines of file
  as list of strings ltvargt open(ltnamegt,
  w') open a file for output ltfile
  variablegt.write(ltstringgt) write ltstringgt to the
  file ltfile variablegt.close() close the file

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Statements

• assignment a,b,c 1,2,3function
  calls log.write(some text\n)print print(some
  fun\n)if/elseif/else conditional
  executionfor/else fixed iterationwhile/else
  indeterminate iterationpass empty statement
  (no-op)break, continue force exit a
  looptry/except/finally catching
  exceptionsraise trigger an exceptionimport,from
  make members of a module accessibledef,return,yi
  eld defining functions

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Statements (cont.)

• class Defining objectsglobal namespacesdel dele
  ting referencesexec running code strings, like
  interpreting a string containing python
  codeassert debugging your codewith/as context
  managers

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