Reading (and writing) files with Python is very easy. Here are some minimalistic code examples for beginners.
Reading
A very common way to handle the contents of a file is by reading the file completely and then working with a single big string. The only reason not to do so is because your file is too big. For your information, this is how long reading a file takes (all times are in seconds, reading is executed 100 times):
file size min max median average
---------------------------------------------------
10 KiB 0.0000 0.0008 0.0000 0.0002
100 KiB 0.0000 0.0009 0.0001 0.0002
1 MiB 0.0003 0.0013 0.0005 0.0006
10 MiB 0.0041 0.0079 0.0047 0.0049
100 MiB 0.0627 0.0778 0.0655 0.0663
As you can see, reading files is quite fast. However, I would recommend the
start thinking if reading the complete file is appropriate when the file
size exceeds 100 MiB as you might get different problems then. For example,
the content of the file might not fit in your main memory (typically 4GiB,
type cat /proc/meminfo | grep MemTotal to get how much main memory your
computer has).
See source code for details how I measured it.
Completely
with open("filename.txt") as f:
content = f.read()
is all you need to read a text file completely in a Python string variable
content. You could, for example, split the whole string to a list of lines:
lines = content.split("\n")
Line by line
You can read a text file line by line, but keep in mind that this will still
have the endline character \n in each line!
with open("filename.txt") as f:
for line in f:
print(line)
Writing
To open files, you have to specify a mode. This is either reading, writing or appending (binary or text data). If you don't specify it, the default value is reading. So for writing you have to specify it:
with open("filename.txt", "w") as f:
f.write("Hello world!")
Python 3
Python 3 has some new features which are very nice to have. To get them in Python 2, you have to add
from __future__ import print_function
at the beginning of your code before other imports happen. Now you can print to files:
from __future__ import print_function
with open("file.txt", "w") as f:
print("Hello World!", file=f)
The print
function has also a "end" parameter which defaults to \n. This means it adds
\n automatically at the end of each printed line. You might not want that
e.g. in a Windows environment where it should be \r\n.
with
You might wonder what
with
does. My advice for newbees would be not to worry too much about it, it is
just the way you juse I/O with Python. If you come from the C / C++ world, you
might know that you have to close files when you opened them. The with
statement makes sure that the file is closed when the block is finished.