Python如何去读取.txt文件中的行并将其拆分为列表？

Question

So I am having trouble understanding how Python creates lists using the .split() string method if I were to give it a file to read.

Here I have a text file with populations from three different countries called population.txt:

United-States 325700000
Canada        37000000
China         13860000000

and in another .py file, I have this code:

populationFile = open("population.txt", 'r')

for populationLine in populationFile:
    populationList = populationLine.split()

print(populationList)

populationFile.close()

The output is this:

['China', '13860000000']

Does python essentially put each country and the respective population in separate lists by reading each line as it did with China, or is it by character? Also, how come only one list appears here and not all of them?

Sorry for all the questions, but I will be very grateful for anyone who can help :)

Answer 1

What you are doing is setting the value for populationList on top of the previous iteration. so it is splitting the United States population, then splitting the Canada population and saving it over the United States, then China replaced Canada.

What you can do it append;

populationFile = open("population.txt", 'r')
populationList = [] # create an empty list

for populationLine in populationFile:
    populationList.append(populationLine.split()) # append the split string into list

print(populationList)

populationFile.close()

If you would like to optimize this, you can use a with block. It would look like this:

with open("population.txt", 'r') as populationFile:
    populationList = [] # create an empty list

    for populationLine in populationFile:
        populationList.append(populationLine.split()) 

print(populationList)

This only opens the file temporarily and when the with block is complete, it closes it automatically.

Answer 2

You need to change your code to this

populationFile = open("population.txt", 'r')

temp = None   
# create an empty list
populationList = []

for line in populationFile:
    # split into different words by the space ' ' character
    temp = line.split()  # temp = ['Canada', '37000000'] or ['China', '13860000000']

    # if spaces exist on either the right or left of any the elements in the temp list
    # remove them
    temp = [item.strip() for item in temp[:]]

    # append temp to the population list
    populationList.append(temp)

print(populationList)

populationFile.close()

Answer 3

how come only one list appears here and not all of them?

populationList is changing after each iteration and losing (by overwriting) its earlier value.

Instead you should try this:

for populationLine in populationFile:
    populationList.append(populationLine.split())