Reading and Writing Files on Your Computer

Transcription

1 Reading and Writing Files on Your Computer

2 Code Snippets HW2-3, HW2-4

3 Function Recap #!/usr/bin/env python3 Though it s called sentence in main, in replace_hello() that value is called text def replace_hello(text): pirate_text = text.replace( hello, ahoy ) return pirate_text def main(): sentence = input( Yarr, type yer words ) sentence_lower = sentence.lower() translated_text = replace_hello(sentence_lower) print(translated_text) if name == main : main() The return of the function is assigned to a variable translated_text

4 Function Recap #!/usr/bin/env python3 Any indented code after a return will not be executed def replace_hello(text): pirate_text = text.replace( hello, ahoy ) return pirate_text def main(): sentence = input( Yarr, type yer words ) sentence_lower = sentence.lower() translated_text = replace_hello(sentence_lower) print(translated_text) if name == main : main() These two lines of code need to be at the bottom of your program

5 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) if name == " main ": main() def out_of_reach(a,b): return a * b $ python3 test.py Traceback (most recent call last): File "test.py", line 10, in <module> main() File "test.py", line 7, in main print(out_of_reach(2,4)) NameError: name 'out_of_reach' is not defined

6 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) if name == " main ": main() def out_of_reach(a,b): return a * b

7 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b When a def is encountered, instead of executing the lines of code, it will compile the code and save it for later def main(): print(out_of_reach(2,4)) if name == " main ": main() def out_of_reach(a,b): return a * b

8 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) Compile main() and save for later if name == " main ": main() def out_of_reach(a,b): return a * b

9 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) if name == " main ": When you run your program, it will execute these two lines after all lines above have been evaluated main() def out_of_reach(a,b): return a * b

10 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) Python will now find the compiled main() function it saved previously if name == " main ": main() def out_of_reach(a,b): return a * b

11 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): Python will look for the function out_of_reach(), but it actually hasn t encountered it yet print(out_of_reach(2,4)) if name == " main ": main() def out_of_reach(a,b): return a * b $ python3 test.py Traceback (most recent call last): File "test.py", line 10, in <module> main() File "test.py", line 7, in main print(out_of_reach(2,4)) NameError: name 'out_of_reach' is not defined

12 Function Recap #!/usr/bin/env python3 def in_reach(a,b): return a + b def main(): print(out_of_reach(2,4)) PUT THESE TWO LINES AT THE if name == " main ": main() BOTTOM OF YOUR FILE TO AVOID THIS PROBLEM def out_of_reach(a,b): return a * b

13 Why we use functions Functions allow you to re-use code that you may use many times Functions help modularize your code, so you can reason about a smaller part of your code at any one time Putting all your code into functions helps reduce bugs in your program Variable names used in one function won t conflict with variables named the same thing in another function You can test each function individually to check if it works correctly Critically analyzing a concrete block of code in a function is easier than a long block of code

14 Modules Libraries provide functionality written by others for use within our own program To make use of these libraries, we import their functionality which are built in modules Python has a LOT of core modules, you can see a list of them at Installing Anaconda initially, gave us even more modules we can use

15 Importing a module We have already used random, and we briefly looked at math and docs.python.org/3/library/math.html #! /usr/bin/env python3 import random def get_computers_choice(): computers_choice = random.choice([ rock, paper, scissors ]) return computers_choice

16 sys module The sys module provides access to interactions with the python interpreter, including information about how the program was started #! /usr/bin/env python3 import sys def main(): filename = input( What file would you like to analyze? ) if.txt not in filename: print(filename + is invalid ) sys.exit() # Program quits execution # Other program code

17 Command-line arguments and sys.argv The words that are typed after your Python program s name in the terminal are called command-line arguments They are available at sys.argv This provides a new modality of input for your programs, input specified at the time the program is run

18 sys.argv Command Argument

19 sys.argv Command Argument

20 sys.argv Command Argument

21 sys.argv sys.argv is a collection of items called a list #!/usr/bin/env python3 import sys print(sys.argv) $ python3 program.py argument1 argument2 argument3 [ program.py, argument1, argument2, argument3 ]

22 sys.argv Get each individual argument using list indexing #!/usr/bin/env python3 import sys print(sys.argv) $ python3 program.py argument1 argument2 argument3 [ program.py, argument1, argument2, argument3 ] sys.argv[0] sys.argv[1] sys.argv[2] sys.argv[3]

23 sys.argv #!/usr/bin/env python3 import sys def main(): kung_fu_panda.txt filename = sys.argv[1] if name == main : main() $ python program.py kung_fu_panda.txt

24 os Provides system-independent operating system functionality Remember a path directs the system where to find a file It is a combination of the directories that contain the file and the filename itself os.path provides useful functionality for paths

25 os.path.exists(path_name) This function will return a boolean that is True if it exists on your computer, otherwise it will return False #!/usr/bin/env python3 import sys def main(): filename = sys.argv[1] if not os.path.exists(filename): print( The file + filename + does not exist ) sys.exit() if name == main : main()

26 Reading and Writing files

27 Computer storage VS Large, slow, permanent Small, fast, temporary

28 The input() function is fairly impractical #!/usr/bin/env python3 import sys def main(): script_text = input( Type in the Kung Fu Panda script, in it s entirety ) if name == main : main()

29 Hard drive storage vs memory With our programs up to today, we have been storing data to variables, this occurs in our computer s temporary memory The computer does not remember this information after your program is run Your computer can also permanently store information on its hard drive Saving your program s file is an example of permanently writing your file to the hard drive Reading and writing files from/to the hard drive represents another input/ output method for our programs

30 Opening/Closing file For both reading and writing, you need to first open a file for reading/writing After you are done reading/writing your program needs to close it When writing to a file, if you don t close your file, the changes you made might not be saved After closing the file, you can t read or write from it again without opening it again

31 Opening/Closing file Opens the file for reading File reference with open(filename, r ) as input_file: text = input_file.read() Don t forget the colon!

32 The file vs filename The filename is just a string that represents the path to the file from the location you ran your program from The file reference connects the program to the file and its contents on the computer filename is a string with open(filename, r ) as input_file: text = input_file.read() input_file refers to the actual file on the computer

33 Reading from a file Use the read() method to read all the text from a file as a complete string with open(filename, r ) as input_file: text = input_file.read() print( The text of the file is + text) After you are done with reading, unindent your code After unindenting, file is automatically closed. The file reference is no longer accessible

34 Writing to a file Use the write() method to write all the text to a file with open(filename, w ) as output_file: print( Writing to + filename) output_file.write(finished_text) # rest of program continues on After you are done with reading, unindent your code After unindenting, file is automatically closed. The file reference is no longer accessible

35 Reading/Writing to file demo

36 Your Homework Complete your program which counts the number of words and sentences in a text Instead of using the input() function, it will load files from the computer For the purpose of this assignment, periods, question marks, exclamation points and ellipses with four dots denote sentence separators Ellipses ( ) don t divide sentences, unless there are four dots (.) Your program will output an answer, but for a long document like the Kung Fu Panda script how do you verify that the answer is correct?

37 Writing test cases for your program You will be asked to write multiple test cases for your program to verify it s functionality These test cases will give you confidence the answer produced by your program is correct

38 Objectives for writing test cases Write test cases that you can manually inspect the correct answer Write the easiest test case you can think of, always run that first Write subsequently more difficult test cases and test those in succession

39 Determining Corner Cases Corner cases are specific test cases that are more likely to fail for incorrectly written programs They generally represent uncommon, but still plausible use cases In your rock, paper, scissors program rock scissors is an example of a corner case, depending on how it s written some programs may interpret this as a rock or a scissors Choose corner cases that you think will likely cause your program to fail, then correct your code until they run correctly

40 Things to test for Non-existent files -> program should produce an error Empty files -> counts should be 0 Files without any punctuation -> Sentence count should be 1 Files with multiple types of punctuation.how many sentences should this be? We don t require your program to correctly consider periods that are part of titles (Dr., Mr., Ms.) or abbreviations (extra credit opportunity!)

41 str.split(separator) An input string is split into smaller strings at each separator The separator can be any string >>> sentence = This is a full sentence. >>> words = sentence.split() >>> print(words) ['This', 'is', 'a', 'full', 'sentence.'] >>> sentence = >>> numbers = sentence.split( - ) >>> print(numbers) [ 456, 186, 413, 389 ]

42 str.split(separator) split returns a list of strings and you can get the number of items by calling length on the result You can get the number of items in the list by using the len() function >>> sentence = This is a full sentence. >>> words = sentence.split() >>> num_words = len(words) >>> print(num_words) 5 >>> sentence = >>> numbers = sentence.split( - ) >>> num_numbers = len(numbers) >>> print(num_numbers) 4

43 Homework hint We haven t talked about lists yet (next class) We don t expect you to do any manipulations on the list returned by str.split() to get the correct answer, except using len() A couple of clever uses of str.replace() to preprocess your text will allow you to only call str.split() once to count sentences, similarly for counting words