Read / write files

Read / write files#

Built-in function open() and context manager with

Read / write files#

There are a lot of specialized tools to open specific file types (images, xml, csv, hdf5, netcdf, etc.). Here we focus on the low-level general method to open files.

open built-in function and file object#

file = open("../common/examples/helloworld.py")
txt = file.read()
file.close()
print(txt)

print("Hello world")

name = "Pierre"
print("My name is " + name)

But what if something weird happens while the file is open (e.g. a division by 0)?

-> Exception is raised that could be caught and run some code that is not aware of the file being open.

-> The file remains open.

Context: with keyword#

For such objects that need to be closed, it is a good practice to use the keyword with. Like this, we are sure that the file will be closed even if there is an error:

with open("../common/examples/helloworld.py") as file:
    txt = file.read()
print(txt)

print("Hello world")

name = "Pierre"
print("My name is " + name)

Important

This is much better than using the close function: use with!

Loop over lines#

with open("../common/examples/helloworld.py") as file:
    for line in file:
        print(f"line ?: " + line.strip())

line ?: print("Hello world")
line ?: 
line ?: name = "Pierre"
line ?: print("My name is " + name)

And now using enumerate to get the index of the line:

with open("../common/examples/helloworld.py") as file:
    for i, line in enumerate(file):
        print(f"line {i:2d}: {line.strip()}")

line  0: print("Hello world")
line  1: 
line  2: name = "Pierre"
line  3: print("My name is " + name)

Options of the built-in function open (read, write, append)#

# write data in a file

with open("/tmp/zoo.txt", "w") as file_zoo:
    file_zoo.write("sam;cat;2\n")
    file_zoo.write("liloo;lion;2\n")

with open("/tmp/zoo.txt", "a") as file_zoo:
    file_zoo.write("peter;panda;5\n")

with open("/tmp/zoo.txt") as file_zoo:
    print(file_zoo.read())

sam;cat;2
liloo;lion;2
peter;panda;5

with open("/tmp/zoo.txt", "r") as file_zoo:
    print(file_zoo.readline())
    print(file_zoo.read())

sam;cat;2

liloo;lion;2
peter;panda;5

Difference between write and print

write writes the raw string in the file: as long as no carriage return (\n) is added, write will write on the same line.

print prints the string to standard output and adds a carriage return at the end. This is why sam;cat;2 is followed by two carriage returns: one from the line (raw: "sam;cat;2\n") and one added by print.

Options of the built-in function open (binary file)#

Until now, we have only written text files. It can of course be much more efficient to use binary format.

with open("/tmp/test", "wb") as file:
    file.write(b"a")

Remarks:

  • In practice, saving data in binary file is most of the time a bad idea. There are much better solutions to do this (see for example h5py and h5netcdf).

  • There are Python libraries to read and process many types for files (csv, xml, json, images, tabulars, etc.).

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