Python Reading Files
In this page:
with open("file_name", "r") as file:
content = file.read()Reading Entire File
f.read() loads an entire file's contents into one string in memory at once -- convenient for small files, but risky for very large ones since the whole thing has to fit in RAM before your program can even start processing it.
Example: Reading Entire File
with open("data.txt", "w") as f:
f.write("all content at once")
with open("data.txt", "r") as f:
print(f.read()) # loads the whole file into one string in memoryReading Line by Line
Looping directly over a file object ('for line in f:') reads one line at a time without loading the whole file into memory first, making it the memory-efficient choice for processing logs or datasets that might be gigabytes in size.
Example: Reading Line by Line
with open("data.txt", "w") as f:
f.write("line1\nline2\n")
with open("data.txt", "r") as f:
for line in f: # reads one line at a time, without loading the whole file first
print(line.strip())Reading All Lines into a List
f.readlines() reads the entire file and returns it as a list of strings, one per line -- unlike the line-by-line loop, this does load everything into memory, but gives you the convenience of indexing into a specific line number afterward.
Example: Reading All Lines into a List
with open("data.txt", "w") as f:
f.write("a\nb\nc\n")
with open("data.txt", "r") as f:
lines = f.readlines() # loads every line into a list of strings
print(lines)Safe Reading of Missing Files
Attempting to open a path that doesn't exist raises FileNotFoundError, so production code typically wraps file-opening in a try/except block to show a clear error message rather than letting the program crash with a raw traceback.
Example: Safe Reading of Missing Files
try:
with open("missing.txt", "r") as f:
print(f.read())
except FileNotFoundError: # raised because the path doesn't exist
print("File not found")Stripping Newline Characters
Lines read from a file retain their trailing '\n' newline character, which is why you'll often see line.strip() or line.rstrip('\n') applied immediately after reading, to clean the text up before further processing.
Example: Stripping Newline Characters
with open("data.txt", "w") as f:
f.write("hello\n")
with open("data.txt", "r") as f:
line = f.readline()
print(repr(line)) # shows the trailing \n that was read from the file
print(repr(line.strip())) # newline removed- Calling
read()on a huge file, which loads everything into memory, when looping over the file reads line by line. - Forgetting that lines keep the trailing
\n, so comparisons fail until you useline.strip(). - Reading from a file object a second time and getting an empty string, because the cursor is at the end; call
seek(0)first.
Chapter Quiz — Complete all 6 topics to unlock
0/6 topics done
Complete these topics first: