Learning Python: Day 31

Relative and absolute file paths

when you pass a filename like pid_digits.txt to Path, Python checks the directory where the file that’s currently being executed is stored
sometimes the file you want to open is stored in a different directory
for example, you might store your program files in a folder called python_work; inside python_work, you might have a folder called text_files to distinguish your program files from the text files they’re manipulating
even though text_files is in python_work, passing Path the name of a file in text_files won’t work, since Python will only look in python_work
need to provide the correct path to get Python to open files from a directory other than the one where your program file is stored
two ways to specify paths in programming
relative file path – tells Python to look for a given location relative to the directory where the current program file is stored
since text_files is in python_work, we need to build a path that starts with the directory text_files, and ends with the filename

path = Path(‘text_files/filename.txt’)

absolute file path – tell Python exactly where the file is on your computer, regardless of where the current program running is stored
alternative if a relative path doesn’t work
for example, if text_files is in a folder that’s not python_work, then passing Path ‘text_files/filename.txt’ won’t work since Python is only checking inside python_work
absolute paths are longer since they start at your system’s root folder

path = Path(‘/home/eric/data_files/text_files/filename.txt’)

for now, store files in the same directory as your program files, or in a folder such as text_files within the directory that stores your program files

Accessing a file’s lines

you’ll often want to examine each line in a file
for example, you might want to read through a file of weather data and work with any line that includes the word sunny
or in a news report, you would look for a line with the tag <headline> and rewrite that line with a specific kind of formatting
splitlines() method – to turn a long string into a set of lines, then use a for loop to examine each line from a file

file_reader.py

from pathlib import Path

path = Path(‘pi_digits.txt’)

contents = path.read_text()

lines = contents.splitlines()

for fine in lines:

….print(line)

we start out by reading the entire contents of the file
if you’re planning to work with the individual lines in a file, you don’t need to strip any whitespace when reading the file
splitlines() method returns a list of all lines in the file
then we assign this list to the variable lines
then loop over these lines and print each one

3.1415926535

8979323846

2643383279

since we haven’t modified any of the lines, the output matches the original text file exactly

Working with a file’s contents

after you’ve read the contents of a file into memory, you can do whatever you want with the data
first, we’ll attempt to build a single string containing all the digits with no whitespace

pi_string.py

from pathlib import Path

path = Path(‘pi_digits.txt’)

contents = path.read_text()

lines = contents.splitlines()

pi_string = “

for line in lines:

….pi_string += line

print(pi_string)

print(len(pi_string))

3.1415926536 8979323846 264338379

variable pi_string contains the whitespace that was on the left side of the digits in each line, but we can use lstrip()

–snip–

for line in lines:

….pi_string += line.lstrip()

print(pi_string)

print(len(pi_string))

now we have a string containing pi to 30 decimal places
string is 32 characters long because it also includes the leading 3 and a decimal point

3.141592653589793238462643383279

Python interprets all text in a text file as a string, if you read in a number and want to work with that value in a numerical context, you’ll have to convert it to an integer using the int() function or a float using the float() function

End of study session.

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