CS116 - Module 10 - File Input/Output

Transcription

1 CS116 - Module 10 - File Input/Output Cameron Morland Winter 2018 Reminder: if you have not already, ensure you: Read Think Python, chapters 8, 12, Cameron Morland CS116 - Module 10 - File Input/Output

2 Creating a text file Using your code editor, create a file somenums.txt and write the following text into it: Cameron Morland CS116 - Module 10 - File Input/Output

3 Template for reading files # open the file f = open(" somenums. txt", "r") line = f. readline() # read first while line!= "": # do something with the string #... line = f. readline() # read next # When you reach the end of the file, # f. readline will return "". # close the file f.close() Starting from this template, write a program which computes the sum of the numbers in somenums.txt Starting from this template, write a program which counts how many times the letter e appears in check.py Edit this program so it counts how many times the word def appears in check.py 4 Cameron Morland CS116 - Module 10 - File Input/Output

4 Template for writing files # open the file f = open("written.txt", "w") # write something to the file f. write(" Hello world!\n") # close the file f.close() Starting from this template, write a program which writes the natural numbers from 0 to 100 to natnums.txt, one on each line. You cannot read from a file opened for writing, or write to a file opened for reading. If you omit the close() function, usually nothing bad will happen. But close your file anyway! 5 Cameron Morland CS116 - Module 10 - File Input/Output

5 The Design Recipe and Files The Purpose should describe what is read from/written to a file. A filename is a Str, not a file. open returns an object of type File. The Effects should mention reading/writing, but details are not required. For testing, use check Testing reading a file: import check # filesum( filename) return sum of Ints # stored in the file named filename. # filesum: Str -> Int # Effects: reads from a file def filesum( filename): f = open( filename, "r") line = f. readline() total = 0 while line!= "": total += int( line) line = f. readline() f.close() return total check. expect(" sum",filesum('somenums. txt'),35) 6 Cameron Morland CS116 - Module 10 - File Input/Output

6 Testing Writing Files Create (by hand) a file that contains the desired output, with a different name. Use check.set_file_exact(actual, expected), then use check.expect to actually do the check. import check # count( filename, n) count from 0 to n-1, and write the # results in the file named filename. # count: Str Nat -> None # Effects: writes to a file def count( filename, n): f = open( filename, 'w') for i in range(n): f. write( str(i) + '\n') f.close() # You must manually create test_toten. txt check. set_file_exact(" toten. txt", " test_toten. txt") check. expect("t0: toten", count(" toten. txt", 10), None) 7 Cameron Morland CS116 - Module 10 - File Input/Output

7 Reading all lines at once using f.readlines() Type in the following program (in the same directory as before, which should still contain a file called somenums.txt), and run it. f = open('nums.txt', 'r') lines = f. readlines() f.close() print( lines) Here we see that f.readlines() returns a list, where each item is a line from the file: lines = ['42\n', '6\n', '7\n'] It is generally a good idea to avoid using readlines(), since it loads the whole file into memory. If your file is really big, you may run out of memory. 9 Cameron Morland CS116 - Module 10 - File Input/Output

8 Writing multiple lines at once using f.writelines() Type in the following program, and run it. Then take a look at written.txt. f = open("written.txt", "w") lines = [ "Hello ", "world!\n", "How ", "are ", "you?\n" ] f. writelines( lines) f.write("bye!\n") f.close() Each item in the list gets written. no newlines are added, only those included in the strings. Hello world! How are you? Bye! 10 Cameron Morland CS116 - Module 10 - File Input/Output

9 More File Practice Download the US database of top baby names for 2016: yob2016.txt (The full dataset for all years is available from the Social Security Administration.) These files contain comma sepated values: Emma,F,19414 Olivia,F,19246 Ava,F,16237 Sophia,F,16070 Isabella,F,14722 Mia,F,14366 Charlotte,F,13030 Abigail,F,11699 Write a function pop_name(fname, n) that consumes a Str of the file name, and a Str of a person name. It returns the popularity of that name. If the name is not in the database, return zero. pop_name('yob2016.txt', 'Mia') => pop_name('yob2016.txt', 'Zaphod') => 0 Write a function pop_range(fname, lo, hi). It returns a (listof Str) of all names used at least lo and not more than hi times. pop_range('yob2016. txt', 1000, 1005) => ['Leia', 'Rylie', 'Angel', 'Dante'] 11 Cameron Morland CS116 - Module 10 - File Input/Output

10 Error Handling Sometimes code will throw an error. Examples: Type mismatch: 5 + "hello" Division by zero: 1 / 0 Trying to open for reading a file which does exist Trying to open for writing a file without permission Usually it is better to avoid these errors by only doing things which won t fail. But this is not always possible or convenient. We can handle these errors using the try and except keywords, like this: try: #... some code which might # throw an error except: # what to do when an error # is thrown 12 Cameron Morland CS116 - Module 10 - File Input/Output

11 Fragile software Fix this flaky program with try/except: def add_reciprocals(l): total = 0 for i in L: total += 1/i return total print( add_reciprocals ([1,3,2])) # print( add_reciprocals ([1,0,2])) A reminder how to use try/except: try: #... some code which might # throw an error except: # what to do when an error # is thrown 13 Cameron Morland CS116 - Module 10 - File Input/Output

12 Handling errors in file handling If a file does not exist, you cannot open it for reading. open will throw an error. open can fail for writing as well. This function can be used to deal with this. Only proceed with your program if safe_open returns something which is not False. ## safe_open produces False if filename ## could not be opened, and produces ## open file object otherwise ## safe_open: Str Str -> ( anyof File False) def safe_open( filename, mode): try: f = open( filename, mode) return f except: print(" File {0} not opened". format( filename)) return False 14 Cameron Morland CS116 - Module 10 - File Input/Output

13 try/except In general, avoid using this construction. Good ideas: Make the try block as small as possible. Only catch errors when you know the cause. Debug your code completely before using one. Don t use it as a substitute for if! 15 Cameron Morland CS116 - Module 10 - File Input/Output

14 Converting between numbers and characters Characters are stored as bytes. A byte is a collection of 8 bits, so a byte can represent numbers Python uses UNICODE, so characters can actually be bigger. Two key functions: ord(s) requires that s be a string of length 1, and returns the number corresponding to the UNICODE character. For example, ord('a') => 97, and ord('\n') => 10. chr(n) goes the other way, creating a string of length 1 containing the UNICODE character. chr(97) => 'a' and chr(10) => '\n'. Ex. What is chr(128013)? 16 Cameron Morland CS116 - Module 10 - File Input/Output

15 Goals of Module 10 Understand the process of reading from files. Understand the process of writing to files. Be able to use ord and chr to convert between numbers and UNICODE characters. Before we begin the next module, please Read the Wikipedia article on Graph Theory 17 Cameron Morland CS116 - Module 10 - File Input/Output