Write files

From answers to files

Until now, your results stayed in variables, or you printed them. Other people need a file: a CSV file that a spreadsheet program opens, or a short text report.

Write text

open takes a second argument, the mode.

Mode	Meaning
"r"	Read. This is the default.
"w"	Write. Python makes the file. If the file exists, Python empties it first.
"a"	Append. Python adds the new text to the end of the file.

with open("notes.txt", "w", encoding="utf-8") as file:
    file.write("First line\n")
    file.write("Second line\n")

write takes one text, and it does not end the line. You add \n yourself.

Be careful with the mode "w". It deletes the old content of the file, and it does not ask.

Write a CSV file

csv.writer does the reverse of csv.reader. Its method writerow takes a list of values and writes one row. It puts quotation marks around a value that has a comma in it.

import csv

with open("pets.csv", "w", newline="", encoding="utf-8") as file:
    writer = csv.writer(file)
    writer.writerow(["name", "age"])
    writer.writerow(["Biscuit", 4])

newline="" is important here. Without it, the file gets an empty line after each row on some computers.

csv.DictWriter writes dictionaries. You give it the names of the columns.

pets = [{"name": "Biscuit", "age": 4}, {"name": "Pepper", "age": 2}]

with open("pets.csv", "w", newline="", encoding="utf-8") as file:
    writer = csv.DictWriter(file, fieldnames=["name", "age"])
    writer.writeheader()
    writer.writerows(pets)

list(pets[0].keys()) gives the names of the columns from the first row.

Columns in a text report

A person reads a text report, so the columns must line up. An f-string can give a value a fixed width. Write a colon after the value, and then the form.

print(f"[{'Old Mill':<12}]")    # [Old Mill    ]
print(f"[{42:>6}]")             # [    42]
print(f"[{3.14159:>8.2f}]")     # [    3.14]

• <12 puts the value to the left, in 12 characters.
• >6 puts the value to the right, in 6 characters.
• >8.2f puts a number to the right, in 8 characters, with 2 digits after the decimal point.

Text usually goes to the left, and numbers go to the right.

Look at a file that you wrote

In this editor, a file that your code writes exists only while the code runs. It has no tab. To look at the file, read it at the end of your code, and print the text.

with open("notes.txt", encoding="utf-8") as file:
    print(file.read())

On your own computer, the file stays in the folder of your program.

Your turn

main.py loads 18 trips, and it makes the list ranked. Each item of ranked is a pair: the name of a start station, and its number of trips. Do the tasks in order.