Specifications Recall: The Python API Function name Possible - PowerPoint PPT Presentation

Mini-Lecture 8 Specifications

Recall: The Python API Function name Possible arguments Module What the function evaluates to 9/12/18 Specifications 2

Recall: The Python API Function name Possible arguments Module What the function evaluates to • This is a specification § Enough info to use func. § But not how to implement • Write them as docstrings 9/12/18 Specifications 3

Anatomy of a Specification One line description, def greet(n): followed by blank line """Prints a greeting to the name n Greeting has format 'Hello <n>!' Followed by conversation starter. Parameter n: person to greet Precondition: n is a string""" print('Hello '+n+'!') print('How are you?') 9/12/18 Specifications 4

Anatomy of a Specification One line description, def greet(n): followed by blank line """Prints a greeting to the name n More detail about the Greeting has format 'Hello <n>!' function. It may be many paragraphs. Followed by conversation starter. Parameter n: person to greet Precondition: n is a string""" print('Hello '+n+'!') print('How are you?') 9/12/18 Specifications 5

Anatomy of a Specification One line description, def greet(n): followed by blank line """Prints a greeting to the name n More detail about the Greeting has format 'Hello <n>!' function. It may be many paragraphs. Followed by conversation starter. Parameter description Parameter n: person to greet Precondition: n is a string""" print('Hello '+n+'!') print('How are you?') 9/12/18 Specifications 6

Anatomy of a Specification One line description, def greet(n): followed by blank line """Prints a greeting to the name n More detail about the Greeting has format 'Hello <n>!' function. It may be many paragraphs. Followed by conversation starter. Parameter description Parameter n: person to greet Precondition: n is a string""" Precondition specifies print('Hello '+n+'!') assumptions we make about the arguments print('How are you?') 9/12/18 Specifications 7

Anatomy of a Specification One line description, def to_centigrade(x): followed by blank line """Returns: x converted to centigrade More detail about the Value returned has type float. function. It may be many paragraphs. Parameter x: temp in fahrenheit Parameter description Precondition: x is a float""" return 5*(x-32)/9.0 Precondition specifies assumptions we make about the arguments 9/12/18 Specifications 8

Anatomy of a Specification One line description, “Returns” indicates a def to_centigrade(x): followed by blank line fruitful function """Returns: x converted to centigrade More detail about the Value returned has type float. function. It may be many paragraphs. Parameter x: temp in fahrenheit Parameter description Precondition: x is a float""" return 5*(x-32)/9.0 Precondition specifies assumptions we make about the arguments 9/12/18 Specifications 9

Preconditions >>> to_centigrade(32.0) • Precondition is a promise § If precondition is true, 0.0 the function works >>> to_centigrade(212) § If precondition is false, 100.0 no guarantees at all • Get software bugs when § Function precondition is not documented properly § Function is used in ways that violates precondition 9/12/18 Specifications 10

Preconditions >>> to_centigrade(32.0) • Precondition is a promise § If precondition is true, 0.0 the function works >>> to_centigrade(212) § If precondition is false, 100.0 no guarantees at all >>> to_centigrade('32') • Get software bugs when Traceback (most recent call last): § Function precondition is File "<stdin>", line 1, in <module> not documented properly File "temperature.py", line 19 … § Function is used in ways TypeError: unsupported operand type(s) that violates precondition for -: 'str' and 'int' Precondition violated 9/12/18 Specifications 11

String Extraction Example def firstparens(text): >>> s = 'Prof (Walker) White' """Returns: substring in () >>> firstparens(s) Uses the first set of parens 'Walker’ Param text: a string with ()""" >>> t = '(A) B (C) D' # Find the open parenthesis >>> firstparens(t) 'A' # Store part AFTER paren tail = s[start+1:] # Find the close parenthesis # Return the result return tail[:end] 9/10/18 Defining Functions 12

String Extraction Example def firstparens(text): >>> s = 'Prof (Walker) White' """Returns: substring in () >>> firstparens(s) Uses the first set of parens 'Walker’ Param text: a string with ()""" >>> t = '(A) B (C) D' # Find the open parenthesis >>> firstparens(t) start = introcs.index_str(s,'(') 'A' # Store part AFTER paren tail = s[start+1:] # Find the close parenthesis tail,')') # Return the result return tail[:end] 9/10/18 Defining Functions 13

String Extraction Example def firstparens(text): >>> s = 'Prof (Walker) White' """Returns: substring in () >>> firstparens(s) Uses the first set of parens 'Walker’ Param text: a string with ()""" >>> t = '(A) B (C) D' # Find the open parenthesis >>> firstparens(t) start = introcs.index_str(s,'(') 'A' # Store part AFTER paren tail = s[start+1:] # Find the close parenthesis ,')') # Return the result return tail[:end] 9/10/18 Defining Functions 14

String Extraction Example def firstparens(text): >>> s = 'Prof (Walker) White' """Returns: substring in () >>> firstparens(s) Uses the first set of parens 'Walker’ Param text: a string with ()""" >>> t = '(A) B (C) D' # Find the open parenthesis >>> firstparens(t) start = introcs.index_str(s,'(') 'A' # Store part AFTER paren tail = s[start+1:] # Find the close parenthesis end = introcs.index_str(tail,')') # Return the result return tail[:end] 9/10/18 Defining Functions 15

String Extraction Example def firstparens(text): >>> s = 'Prof (Walker) White' """Returns: substring in () >>> firstparens(s) Uses the first set of parens 'Walker’ Param text: a string with ()""" >>> t = '(A) B (C) D' # Find the open parenthesis >>> firstparens(t) start = introcs.index_str(s,'(') 'A' # Store part AFTER paren tail = s[start+1:] # Find the close parenthesis end = introcs.index_str(tail,')') # Return the result return tail[:end] 9/10/18 Defining Functions 16

String Extraction Example def second(thelist): >>> second('cat, dog, mouse, lion') """Returns: second elt in thelist 'dog' Ex: second('A, B, C') => 'B' >>> second('apple, pear, banana') Param thelist: a list of words 'pear' Precond: thelist has words sep. by commas, spaces.""" 9/12/18 Specifications 17

String Extraction Example def second(thelist): >>> second('cat, dog, mouse, lion') """Returns: second elt in thelist 'dog' Ex: second('A, B, C') => 'B' >>> second('apple, pear, banana') Param thelist: a list of words 'pear' Precond: thelist has words sep. by commas, spaces.""" # Find start of second elt # Find end of second elt # Slice from start to end # Return result 9/12/18 Specifications 18

String Extraction Example def second(thelist): >>> second('cat, dog, mouse, lion') """Returns: second elt in thelist 'dog' Ex: second('A, B, C') => 'B' >>> second('apple, pear, banana') Param thelist: a list of words 'pear' Precond: thelist has words sep. by commas, spaces.""" # Find FIRST comma # Find SECOND COMMA # Slice from comma to comma # Return result 9/12/18 Specifications 19

String Extraction Example def second(thelist): >>> second('cat, dog, mouse, lion') """Returns: second elt in thelist 'dog' Ex: second('A, B, C') => 'B' >>> second('apple, pear, banana') Param thelist: a list of words 'pear' Precond: thelist has words sep. by commas, spaces.""" s = introcs.index_str(thelist,',’) e = introcs.index_str(thelist,',',s+1) result = thelist[s+1:e] return result 9/12/18 Specifications 20

String Extraction Example def second(thelist): >>> second('cat, dog, mouse, lion') """Returns: second elt in thelist 'dog' Ex: second('A, B, C') => 'B' >>> second('apple, pear, banana') Param thelist: a list of words 'pear' Precond: thelist has words sep. by commas, spaces.""" Where is the error? s = introcs.index_str(thelist,',’) A: Line 1 e = introcs.index_str(thelist,',',s+1) B: Line 2 result = thelist[s+1:e] C: Line 3 return result D: Line 4 E: There is no error 9/12/18 Specifications 21