循环打印多行（按预期），但只向文件写入一行

Question

The loop reads all text files in the directory and prints the value from the first line of each text file (as expected) but only writes one line to the file.

I have tried several variants of the loop using for and while loops but I think I may be doing them wrong. I am thrown off because it prints the correct output (several lines) but only writes one line.

# this program reads the files in this directory and gets the value in the first line of each text file
# it groups them by the first two numbers of their filename

import glob

# get list of all text files in directory
path = "./*.txt"
txt_files = glob.glob(path)

# these are the groups I need to sort the results by
a1 = "10"
a2 = "20"
b1 = "30"
c1 = "40"

# list of files is in txt_files

for fileName in txt_files:

    # get the two digits from the filename to group the files
    device = fileName[2:4]

    # if the file name's first two digits (device) match the variable, open the file and get the value in the first line

    if device == a1:
        file = open(fileName)
        line = file.readline()
        # then, write that first line's value to the usage.txt file
        print(device + "_" + line)
        fileU = open("usage.txt", 'w')
        fileU.write(device + "_" + line + "\n")
        file.close()

    # if the file name's first two digits = 20, proceed

    elif device == a2:
        # open the text file and get the value of the first line
        file = open(fileName)
        line = file.readline()
        print(device + "_" + line)
        fileU = open("usage.txt", 'w')
        fileU.write(device + "_" + line + "\n")
        file.close()

    # if the file name's first two digits = 30, proceed

    elif device == b1:
        file = open(fileName)
        line = file.readline()
        print(device + "_" + line)
        fileU = open("usage.txt", 'w')
        fileU.write(device + "_" + line + "\n")
        file.close()

The expected results would be usage.txt showing the same output as what is printed in the console.

usage.txt will only have one line: 30_33

the console will print all of the lines:

10_36

10_36

20_58

20_0

20_58

30_33

30_33

Process finished with exit code 0

Answer 1

You are opening and truncating the file, open with append :

Since you open the file each time you loop, and you are not using a , you are truncating the file each loop so each loop you write a new 1 line file.

fileU = open("usage.txt", 'a')

Answer 2

You're recreating the file each time you open it in the loop. You should open it once before the loop.

with open("usage.txt", "w") as fileU:
    for fileName in txt_files:

        # get the two digits from the filename to group the files
        device = fileName[2:4]

        # if the file name's first two digits (device) match the variable, open the file and get the value in the first line

        if device == a1:
            file = open(fileName)
            line = file.readline()
            # then, write that first line's value to the usage.txt file
            print(device + "_" + line)
            fileU.write(device + "_" + line + "\n")
            file.close()

        # if the file name's first two digits = 20, proceed

        elif device == a2:
            # open the text file and get the value of the first line
            file = open(fileName)
            line = file.readline()
            print(device + "_" + line)
            fileU.write(device + "_" + line + "\n")
            file.close()

        # if the file name's first two digits = 30, proceed

        elif device == b1:
            file = open(fileName)
            line = file.readline()
            print(device + "_" + line)
            fileU.write(device + "_" + line + "\n")
            file.close()