Key Ideas

  • Opening a File

    • To read from it: file_variable = open(filename, "r")
    • To write to it: file_variable = open(filename, "w")
    • To close it: file_variable.close()

  • Absolute File Paths vs. Relative File Paths

  • The file census.txt captures this 2022 census data.

  • Now, let’s write some code to open and read census.txt:

First, we need to create a “file object”, which is a Pythonic way of saying we need to open the file and store its contents in a variable. This is done with the open() command, which requires as parameters 1) the path to the file we want to open and 2) the read/write option we want to use with the file.

  census_file = open("census.txt", "r")
  
  census_file.close()

Next, we use a for loop to work our way through the file one line at a time. There are several methods for doing this, we we’ll start with this one.

  census_file = open("census.txt", "r")
  
  for one_line in census_file:		# the newline mark triggers our iteration
      print(one_line)
  
  census_file.close()

The program is reading the file data one line at a time, with each line as a single string. To make these lines more “usable”, we want to turn each line into a list with the elements of the state name and the population, like this: ["California", "38332521"]. We’ll use the split() method to do this.

  census_file = open("census.txt", "r")
  
  for one_line in census_file:		# the newline mark triggers our iteration
      values = one_line.split()
      print(values)
  
  census_file.close()

Finally, now that we can “grab” the individual data points in each line, we can print the data in a more user-friendly way:

  census_file = open("census.txt", "r")
  
  for one_line in census_file:		# the newline mark triggers our iteration
      values = one_line.split()
      print("State: " + values[0] + ", Population: " +  values[1])
  
  census_file.close()

Active Learning

  • Modify census.py so that it calculates and prints the total 2022 U.S. population. (If your program is correct, the number will match the number on the web page above.)
  • Do Exercises 1-3

HW: