CSCA20 Worksheet Working with Files

Transcription

1 CSCA20 Worksheet Working with Files 1 Philosophical question: what s a file? Q. A general answer? Q. A programmer s answer? 2 Opening and closing a file To start using a file, given its filename, you have to open it. But first, the name a string. How we get that string depends on our needs. For example: If we will always read from the same file, then we might want to...? If we want the user to specify it, then we can...? To open the file, use the function open: Form to open a file: myfile = open("story.txt", "r") open is the Python function story.txt is the name of the file to be opened myfile is a variable that is assigned the file object (or stream or reader / writer ) returned by open "r" is a string indicating what we wish to do with the file. Options for this string are "r", "w", "a", meaning read, write or append. The default is "r". Be careful! If you write to a file that already exists, the old contents are erased. If you append, the new content is added to the end of the file. After we are finished reading or writing to a file, we should always close the file: myfile.close() where myfile is the file object originally returned by open. 1

2 3 Reading from a file We call a file object that was opened for reading a reader. Here are five ways to read from a reader: 1. Read lines one at a time from beginning to end: for line in myfile: <statements> Suppose that <statements> is print(line). Then the output is double spaced. Q. Why? Q. How can we single space the output? Two ways: (a) (b) It s common to need to strip white space from either end of a line, or both ends. 2. Read everything in the file at once into a list of strings: # Read the whole file into list str_ls. Each element of str_ls is a line. str_ls = myfile.readlines() print(str_ls) 3. Read everything in the file at once into a string: s = myfile.read() # Read the whole file into string s. 4. Read a certain number of characters: s = myfile.read(10) # Read 10 characters into s. s = myfile.read(10) # Read the next 10 characters into s. 2

3 5. Read a line at a time: s = myfile.readline() # Read a line into s. s = myfile.readline() # Read the next line into s. s = myfile.readline() # And so on... Q. Why would you ever use the line-at-time approach (5) instead of a for loop (1)? 4 Dealing with end of file Unless you choose to stop reading, you will eventually reach the end of an input file, called EOF. With method (1) above, the for loop automatically recognizes EOF. Otherwise, you have to detect the situation of being at EOF yourself. Q. What happens if you are at EOF when you call read() or readline()? Example Write code to open a file called filename and read line by line until EOF (end of file) is reached. Q. Why doesn t this loop stop when you get to any empty line? 5 Writing to a file First we open a file to write: Then we write the contents: output_file.write("dear Uncle Marty,\n") output_file.write("thanks for the nifty present.\n") output_file.write("love,\nyour niece\n") 3

4 Then we close the file: 6 Let s practise! Write functions to: Prompt the user for a file name, open the file, and return the opened reader. def prompt_and_open(): (NoneType) -> file Prompt the user to enter a file name, open the file for reading and return the open file object. Read the file and print all lines that start with a specified character: def display_starts_with(our_file, ch): (file, str) -> NoneType Print the lines of our_file, an open reader, that start with the character ch. Now we can write a program using these two functions: if name == main : input_file = prompt_and_open() display_starts_with(input_file, "#") input_file.close() Q. Our program misses one of the comments in grades.csv. Why? Q. How can we fix this? 4

5 7 CSV files The CSV (comma-separated values) format is commonly used to transfer data between locations, computers or applications. For example, suppose we have a spreadsheet that looks like this: One two three 10,326 Ronald "The Gipper" Reagan "Hi, mom!" said Jim. That s all. If we save it in a CSV file, the result looks like this: One,two,three "10,326","Ronald ""The Gipper"" Reagan", """Hi, mom!"" said Jim.",That s all., What s good about CSV? All spreadsheets understand it: they can write it and read it. (If you find one that can t, avoid using that spreadsheet application.) You can find CSV functions in many programming languages, for example Python s csv module. This means you too can read and write CSV, even if there s no spreadsheet involved. It s easy to generate CSV on your own, provided you re a little careful if there might be quotation marks and commas in your data. What s not so good about CSV? All spreadsheet formatting is lost font, font size, cell size, and more. Newline characters may differ when moving from one system to another. There may be no newline character at the end of the last line. Applications trying hard to preserve all possible information may generate Unicode (two-byte) characters that can be hard to handle in a simple program. These difficulties are not hard to deal with, and if you have data to transfer from one program to another, CSV may be a good way to do it. 8 Reading CSV in Python Here are the essential elements: import csv... instream = open(file_name) reader = csv.reader(instream)... for line in reader: # Reading as for an ordinary file, but line is a list. 5

6 9 Using urls instead of files In Python opening and reading from a URL is much like opening and reading from a file. We use a module urllib for this purpose. import urllib.request url_reader = urllib.request.urlopen( ) for line in url_reader: print(line, end= ) The output we got is not exactly what we wanted. That s because with URLs, the reader does not return strs. To get the corresponding strs, we can use the method decode: import urllib.request url_reader = urllib.request.urlopen( ) for line in url_reader: print(line.decode(), end= ) 6