Convert strings of key-multiple values into key-value pairs

Question

I have a space-separated text file. Each line has a one item followed by zero or more additional items. All values are strings.

I wish to output the data as key-value pairs.

How can this be done in Python?

Input:

1: 200
2:
3: 300 400 abc
4: xyz 300

Desired Output:

1: 200
2:
3: 300
3: 400
3: abc
4: xyz
4: 300

If it makes it easier, line 2 could be omitted from the output. Output will be sorted by key (1st column).

Code starter:

# Open the text file
file = open("data.txt", "r")

# Read each \n terminated line into a list called 'lines'
lines = file.readlines()

# Iterate through each line
for line in lines:
  # Remove leading/trailing spaces and newline character
  line = line.strip()

  # Split the line into list items (but the number varies with each line)
  .... = line.split(" ")
  .
  .
  ?

Answer 1

Using a simple iteration.

Ex:

result = []
with open(filename) as infile:
    for line in infile:                   #Iterate each line
        key, val = line.split(":")        #Get key-value
        for i in val.strip().split():
            result.append((key, i))

with open(filename, "w") as outfile:      #Write Output
    for line in result:
        outfile.write(": ".join(line) + "\n")  

Output:

1: 200
3: 300
3: 400
3: abc
4: xyz
4: 300