What is the correct format to write float value to file in Python

Question

I have a bunch of float values, for example:

x1 = 1.11111111

x2 = 2.22222222

I want to write these values to a file:

f = open("a.dat", "w+")
f.write("This is x1: ",x1)
f.write("\n")              #I want to separate the 2 lines
f.write("This is x2: ",x2)

At this point I got an error on the second line:

write() takes exactly one argument (2 given)

How do I write to file such that when I open it, I see this format:

This is x1: 1,1111111
This is x2: 2,2222222

And yes, the file has to be ***.dat

It's not .txt

Answer 1

The write function takes a single string. You're trying to use it like print, which takes any number of arguments.

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You can, in fact, just use print. Its output only goes to your program's output (stdout) by default, by passing it a file argument, you can send it to a text file instead:

print("This is x1: ", x1, file=f)

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If you want to use write, you need to format your output into a single string. The easiest way to do that is to use f-strings:

f.write(f"This is x1: {x1}\n")

Notice that I had to include a \n on the end. The print function adds its end parameter to the end of what it prints, which defaults to \n. The write method does not.

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Both for backward compatibility and because occasionally they're more convenient, Python has other ways of doing the same thing, including explicit string formatting:

f.write("This is x1: {}\n".format(x1))

… printf-style formatting:

f.write("This is x1: %s\n" % (x1,))

… template strings:

f.write(string.Template("This is $x1\n").substitute(x1=x1))

… and string concatenation:

f.write("This is x1: " + str(x1) + "\n")

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All but the last of these automatically converts x1 to a string in the same way as str(x1), but also allows other options, like:

f.write(f"This is {x1:.8f}\n")

This converts x1 to a float, then formats it with 8-decimal precision. So, in addition to printing out 1.11111111 and 2.22222222 with 8 decimals, it'll also print 1.1 as 1.10000000 and 1.23456789012345 as 1.23456789.

The same format strings work for f-strings, str.format, and the format functions:

print("This is x1: ", format(x1, '.8f'), file=f)
f.write("This is x1: {:.8f}\n".format(x1))
f.write("This is x1: " + format(x1, '.8f') + "\n")

… and the other two methods have similar, but not quite as powerful, formatting languages of their own:

f.write("This is x1: %.8f\n" % (x1,))