[Python3] Trailing newline in readlines.

[LWFlouisa]

I seem to have having an issue with trailing newlines when reading in a file.

Steps to reproduce the problem:

(1. Create a file with newlines.
(2. Read in file as an array with readlines.
(3. Sample array with random.
(4. Print sample array to screen.

What seems to get printed is:

Hello World\n.

And not:

Hello World. I have to use static data ( directly hard coded data ) in order to use it.

My use case:

I tend to read version numbers from a file, and use that version number to choose an item from an array. I don't want a newline symbol also being printed.

The error is from readlines().

Is it an issue with random, or something else?

This seems to work fine though:

# Import necessary modules.
import os
import random
import time
 
# Clear the screen
os.system("clear")

# Create a datasheet of pet varieties.
pets = [
  "cat",    "dog",     "rat",
  "gerbil", "hamster", "guiniepig",
  "monkey", "snake",
]

# Create a datasheet of interior varieties.
interior = [
  "bedroom",     "closet",  "bathroom",
  "hallway",     "kitchen", "living room",
  "dining room", "garage",
]

# Create a datasheet of exterior varieties.
exterior = [
  "outhouse", "driveway", "frontyard",
  "backyard", "fence",
]

# Create pet string from pet sample stripping brackets.
sample_pets = random.sample(pets, k = 1)
active_pet = str(sample_pets).strip("[]''")

# Create interior string from interior sample stripping brackets.
sample_interior = random.sample(interior, k = 1)
active_interior = str(sample_interior).strip("[]''")

# Create exterior string from exterior sample_interior stripping brackets.
sample_exterior = random.sample(exterior, k = 1)
active_exterior = str(sample_exterior).strip("[]''")

# Display ai result symbolically.
print(active_pet + "(" + active_interior + ", " + active_exterior + ")\n")

file = open("data.txt", "w")

file.write("I have an animal in my"              + active_interior +
           ". The animal within my house is my " + active_pet      +
           ". Therefore, it is my "              + active_pet      +
           " that is in my "                     + active_exterior +
           ".\n")

file.close()

This performs as I expect it without the trailing newlines.

I'm more used to Ruby, where reading in a file doesn't cause the newline to actually show up. I read in files a lot for things like using version numbers to pick an item in an array.

[buran]

this is not an error, but expected behavior with readlines. Every line in the text file will have trailing new line '\n'
use str.strip() method to remove it - iterate over the list returned from random and strip the new lines from every element.

as a side note, this:

active_pet = str(sample_pets).strip("[]''")

is not he way to construct comma separated list of strings. use str.join method

[LWFlouisa]

Figured it out I think.

f = open("array.txt", "r")

display = f.readlines()

print(display[0])

Only wanting to print a part of a list, not show the entire thing.

[buran]

Figured it out I think.

Not really. When printing any of the elements in display you will not see \n at the end like you do in a list, where you see the representation of the str, incl. non-printing chars like new line, tab, etc. But it's there and as a result you will see an extra blank line after the output - one from the string itself and one from the print function.
It really depends what you want to do with the lines from the file - do you just print them (in which case you can just pass end='' argument to print function in which case you will avoid the extra new line. Or you want to process/use them in some way, in which case you need to strip the new line from the end of the string.

sample.txt

Output:
dog
cat

with open('sample.txt') as f:
    content = f.readlines()
print(content)
for line in content:
    print(line)

# how to strip new line
print('\n-----')
with open('sample.txt') as f:
    content = [line.strip() for line in f]

print(content)
for line in content:
    print(line)

Output:
['dog\n', 'cat\n']
dog

cat


-----
['dog', 'cat']
dog
cat

[perfringo]

I observe some things that can be improved (by my subjective opinion).

1. Get random item from list -> use random.choice. Benefits: only one item is needed, returns item (string) right away. Use sample when more than one random item is needed.

>>> pets = ['cat', 'dog', 'rat', 'gerbil', 'hamster', 'guiniepig', 'monkey', 'snake']
>>> random.choice(pets)
'dog'

2. To construct string from different parts -> use f-strings. Benefits: better visual readability, no need for type conversion (if any). Requires 3.6 <= Python.

# Display ai result symbolically.
>>> print(f'{active_pet} ({active_interior}, {active_exterior})\n')

3. Open files -> use with context manager (as in buran examples). Benefits: file will be always properly and automagically closed, no need for close().

with open('data.txt', 'w') as f:

4. Get only first row in file -> use built-in next(). Benefits: whole file content will not be read into memory/list to just get first row. Grab first row from existing fileobject and you are done.

with open('data.txt', 'r') as f:
    my_data = next(f).strip()           # strips newline at the end, can be omitted.