assign.txt - introduce assignment
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Variables and assignment

 Value (data) vs. variable (name with value)

 A variable is a name that refers to a value

 Assignment associates a value with a variable (the name)
   x = 2

 Box diagram shows effect of assignment on memory contents (sects 2.2, 7.1, etc.)

   x ---> 2  All Python variables are references, like pointers (but no arithmetic)

 Assignment creates a variable if it doesn't already exist (no declarations)
  Pitfall: if you misspell on left-hand side, you get a new variable!

 Equality vs identity - not the same - for some types this matters
   x --> 2        x --> 2
   y --> 2   vs.  y _-^     What happens to x and y if you change right-hand side?

 = assigns, == checks equality, is checks identity, id queries identity


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Memory management

 Python automatically allocates and reclaims memory

  s = 'x'*1000000  # Python allocates lots of memory for big string 'xxx...'

  s = 'y'*1000000  # Again for 'yyy...'

 Now 'xxx....' is still in memory but it is unreachable - it is "garbage"

 Python automatically performs "garbage collection" to reclaim memory













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Assignment statements 

 Assignments are statements (not expressions), right-hand side is expression 

  x = 2 + 2  # rhs is expression

 A variable is a kind of expression -- it evaluates to its value
  so a variable can appear in an expression, can appear on rhs of assignment
   
  x = x + 1  # variable x on rhs is expression

 Assignment syntax shortcuts

  x += 1  # same as x = x + 1, similar for -= *= etc.

 (aside: C style, RIP DMR, CPython, Cython ...)

 Python is dynamically typed (unlike statically typed C, C++, Java, ...)
  Values have types, but variables do not
  Can assign values with different types to the same variable
  Tradeoff: dynamically typed programs are shorter BUT compiler does less checking 



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Multiple assigment

 Can assign multiple variables from multiple expressions in one statement

   i, j = 2 + x,  3 * y   # commas separate variables on lhs, exprs on rhs

 Python evaluates all the expressions on the right before doing any assignments

   i, j = j, i            # parlor trick: swap in one statement

 These are just tricks, but multiple assignment is more helpful in other contexts