Compilers and Interpreters

2. Compilers and Interpreters • Programs written in high-level languages must be translated into machine language to be executed • Compiler: translates high-level language program into separate machine language program • Machine language program can be executed at any time

3. Compilers and Interpreters (cont’d.) • Interpreter: translates and executes instructions in high-level language program • Used by Python language • Interprets one instruction at a time • No separate machine language program • Source code: statements written by programmer • Syntax error: prevents code from being translated

4. Compilers and Interpreters (cont’d.) Executing a high-level program with an interpreter

5. compile execute source code Hello.java byte code Hello.class output interpret source code Hello.py output Compiling and interpreting • Many languages require you to compile (translate) your program into a form that the machine understands. • Python is instead directly interpreted into machine instructions.

6. Using Python • Python must be installed and configured prior to use • One of the items installed is the Python interpreter • Python interpreter can be used in two modes: • Interactive mode: enter statements on keyboard • Script mode: save statements in Python script

7. Interactive Mode • When you start Python in interactive mode, you will see a prompt • Indicates the interpreter is waiting for a Python statement to be typed • Prompt reappears after previous statement is executed • Error message displayed If you incorrectly type a statement • Good way to learn new parts of Python

8. Writing Python Programs and Running Them in Script Mode • Statements entered in interactive mode are not saved as a program • To have a program use script mode • Save a set of Python statements in a file • The filename should have the .py extension • To run the file, or script, type python filename at the operating system command line

9. Python Scripts • When you call a python program from the command line the interpreter evaluates each expression in the file • Familiar mechanisms are used to provide command line arguments and/or redirect input and output • Python also has mechanisms to allow a python program to act both as a script and as a module to be imported and used by another python program

10. The IDLE Programming Environment • IDLE (Integrated Development Learning Environment): single program that provides tools to write, execute and test a program • Automatically installed when Python language is installed • Runs in interactive mode • Has built-in text editor with features designed to help write Python programs

11. IDLE Development Environment • IDLE is an Integrated DeveLopment Environ-ment for Python, typically used on Windows • Multi-window text editor with syntax highlighting, auto-completion, smart indent and other. • Python shell with syntax highlighting. • Integrated debuggerwith stepping, persis-tent breakpoints,and call stack visi-bility

12. Programming basics • codeorsource code: The sequence of instructions in a program. • syntax: The set of legal structures and commands that can be used in a particular programming language. • output: The messages printed to the user by a program. • console: The text box onto which output is printed. • Some source code editors pop up the console as an external window, and others contain their own console window.

13. The Basics

14. A Code Sample (in IDLE) x = 34 - 23 # A comment. y = “Hello”# Another one. z = 3.45 if z == 3.45 or y == “Hello”: x = x + 1 y = y + “ World”# String concat. print (x) print (y)

15. Enough to Understand the Code • Indentation matters to code meaning • Block structure indicated by indentation • First assignment to a variable creates it • Variable types don’t need to be declared. • Python figures out the variable types on its own. • Assignment is = and comparison is == • For numbers + - * / %are as expected • Special use of + for string concatenation and % for string formatting (as in C’s printf) • Logical operators are words (and, or, not) not symbols • The basic printing command is print

16. Basic Datatypes • Integers (default for numbers) z = 5 // 2 # Answer 2, integer division • Floats x = 3.456 • Strings • Can use “” or ‘’ to specify with “abc” == ‘abc’ • Unmatched can occur within the string: “matt’s” • Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: “““a‘b“c”””

17. Whitespace Whitespace is meaningful in Python: especially indentation and placement of newlines • Use a newline to end a line of code Use \when must go to next line prematurely • No braces {} to mark blocks of code, use consistent indentation instead • First line with less indentation is outside of the block • First line with more indentation starts a nested block • Colons start of a new block in many constructs, e.g. function definitions, then clauses

18. Comments • Start comments with #,rest of line is ignored • Can include a “documentation string” as the first line of a new function or class you define • Development environments, debugger, and other tools use it: it’s good style to include one def fact(n): “““fact(n) assumes n is a positive integer and returns facorial of n.”””assert(n>0) if n==1 return 1 else n*fact(n-1)

19. Assignment • Binding a variable in Python means setting a name to hold a reference to some object • Assignment creates references, not copies • Names in Python do not have an intrinsic type, objects have types • Python determines the type of the reference automatically based on what data is assigned to it • You create a name the first time it appears on the left side of an assignment expression: x = 3 • A reference is deleted via garbage collection after any names bound to it have passed out of scope • Python uses reference semantics(more later)

20. Assignment • You can assign to multiple names at the same time >>> x, y = 2, 3 >>> x 2 >>> y 3 This makes it easy to swap values >>> x, y = y, x • Assignments can be chained >>> a = b = x = 2

21. Naming Rules • Names are case sensitive and cannot start with a number. They can contain letters, numbers, and underscores. bob Bob _bob _2_bob_ bob_2 BoB • There are some reserved words: and, assert, break, class, continue, def, del, elif, else, except, exec, finally, for, from, global, if, import, in, is, lambda, not, or, pass, print, raise, return, try, while

22. Naming conventions The Python community has these recommend-ed naming conventions • joined_lower for functions, methods and, attributes • joined_lower or ALL_CAPS for constants • StudlyCaps for classes • camelCase only to conform to pre-existing conventions • Attributes: interface, _internal, __private

23. Accessing Non-Existent Name Accessing a name before it’s been properly created (by placing it on the left side of an assignment), raises an error >>> y Traceback (most recent call last): File "<pyshell#16>", line 1, in -toplevel- y NameError: name ‘y' is not defined >>> y = 3 >>> y 3

24. Expressions • expression: A data value or set of operations to compute a value. Examples: 1 + 4 * 3 42 • Arithmetic operators we will use: • + - * / addition, subtraction/negation, multiplication, division • % modulus, a.k.a. remainder • ** exponentiation • precedence: Order in which operations are computed. • * / % ** have a higher precedence than + -1 + 3 * 4 is 13 • Parentheses can be used to force a certain order of evaluation.(1 + 3) * 4 is 16

25. Boolean Operations — and, or, not • Comparisons

26. Numeric Types — int, float

27. Math commands • Python has useful commands for performing calculations. • To use many of these commands, you must write the following at the top of your Python program: from math import *