Creating a new file, filename contains loop variable, python

Question

I want to run a function over a loop and I want to store the outputs in different files, such that the filename contains the loop variable. Here is an example

for i in xrange(10):
   f = open("file_i.dat",'w')
   f.write(str(func(i))
   f.close()

How can I do it in python?

Answer 1

Simply construct the file name with + and str. If you want, you can also use old-style or new-style formatting to do so, so the file name can be constructed as:

"file_" + str(i) + ".dat"
"file_%s.dat" % i
"file_{}.dat".format(i)

Note that your current version does not specify an encoding (you should), and does not correctly close the file in error cases (a with statement does that):

import io
for i in xrange(10):
   with io.open("file_" + str(i) + ".dat", 'w', encoding='utf-8') as f:
       f.write(str(func(i))

Answer 2

Concatenate the i variable to a string as follows:

f = open("file_"+str(i)+".dat","w")

OR

f = open("file_"+`i`+".dat","w") # (`i`) - These are backticks, not the quotes.

See here for other techniques available.

Answer 3

Use f = open("file_{0}.dat".format(i),'w'). Actually, you might want to use something like f = open("file_{0:02d}.dat".format(i),'w'), which will zero-pad the name to keep it at two digits (so you get "file_01" instead of "file_1", which can be nice for sorting later). See the documentation.

Answer 4

Try this:

for i in xrange(10):
   with open('file_{0}.dat'.format(i),'w') as f:
       f.write(str(func(i)))

Answer 5

Use f-strings

• This is the simplest and most recent implementation of string formatting.
• f-Strings: A New and Improved Way to Format Strings in Python
• PEP 498 - Literal String Interpolation
• The string is preceded by an f and the variable is inside the string quotes, surrounded by {}.
  • f"file_{i}.dat"
• Reading and Writing Files
  • The advantage is that the file is properly closed after its suite finishes, even if an exception is raised at some point.
• For python 3, this use range not xrange
• f.write(f'{func(i)}') or f.write(str(func(i)))

def func(i):
    return i**2

for i in range(10):
    with open(f"file_{i}.dat", 'w') as f:
        f.write(f'{func(i)}')