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Python The basics
- 1. Python - The Basics M.BOBBY ASSISTANT PROFESSOR & HEAD, DEPARTMENT OF COMPUTER SCIENCE SRI ADI CHUNCHANAGIRI WOMEN’S COLLEGE, CUMBUM
- 2. 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
- 3. 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
- 4. 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”””
- 5. 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
- 6. 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) return 1 if n==1 else n*fact(n-1)
- 7. 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)
- 8. 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
- 9. 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
- 10. 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
- 11. 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