Handling Whitespace in Strings

startswith() checks if a string begins with a specific prefix and returns True or False—it does not search for the substring anywhere in the string.

Programming

The Hidden Newline

A line read from a file may look like ordinary text, but it commonly includes a newline character at its end. That character is invisible when you look at the string, yet it affects what happens when the line is printed. If print() receives a line that already ends with a newline, the line supplies one newline and print() adds another. The result is an unwanted blank line between displayed lines.

What do you think happens?

A file line contains the text "From: Sam" followed by its newline character. What happens when print() displays that line?

It displays one line with normal spacing
It displays the text with double spacing
It removes the newline automatically
It prints nothing

Reveal answer

Answer:

It displays the text with double spacing

The newline stored in the file line is printed as part of the string, and print() adds its own newline after the string.

What happens when a file line already contains a newline character and print() adds another one?

Checking a Prefix

Definition

The startswith() method checks whether a string begins with a specified prefix. It returns True when the prefix is at the beginning of the string and False otherwise. It does not search for that text anywhere else in the string.

python

line = "From: Sam"

print(line.startswith("From:"))
print(line.startswith("Sam"))
print("Message from Sam".startswith("From:"))

line = "From: Sam"

print(line.startswith("From:"))
print(line.startswith("Sam"))
print("Message from Sam".startswith("From:"))

Output (expected)

True
False
False

The first call returns True because From: is at the beginning of the string. The second call returns False because Sam is not the beginning. The third call also returns False: although the text contains the word from in a later position, startswith() only examines the beginning of the string.

What is the difference between a prefix match at the start of a string and the same text appearing later?

Removing Trailing Characters

Definition

The rstrip() method removes trailing whitespace and newline characters from the right end of a string. It can remove spaces, tabs, and newlines together, which makes it more readable and practical than manually slicing off only the last character.

python

line = "From: Sam\n"
clean_line = line.rstrip()

print(clean_line)
print(clean_line.startswith("From:"))

line = "From: Sam\n"
clean_line = line.rstrip()

print(clean_line)
print(clean_line.startswith("From:"))

Output (expected)

From: Sam
True

In this example, rstrip() produces a new cleaned string without the trailing newline. The visible text remains From: Sam, but printing the cleaned string no longer carries the file line's extra newline into the output.

What changes in a string when rstrip() removes characters from its right end?

Filtering File Lines

A common file-processing workflow is to read lines in a for loop, remove their trailing whitespace with rstrip(), and then test the cleaned line with startswith(). If the test returns True, the line is displayed. This is useful when different lines have different prefixes, such as From:, Subject:, To:, or Date: in email files, or ERROR:, WARNING:, and INFO: in log files.

Worked example

Select From Lines

Given several file lines, display only the lines that begin with From: and avoid extra blank lines.

1
Read one line
The for loop receives a line from the file. The line includes its trailing newline character.
2
Clean the line
Call rstrip() to remove trailing whitespace and newline characters before displaying the line.
3
Check the prefix
Call startswith("From:") to determine whether the cleaned line begins with the required prefix.
4
Display a match
Only a line that passes the prefix check is printed, so unrelated lines are not included.

Result

The workflow extracts the requested From: lines and displays them with single spacing.

Code walkthrough

for line in file:
    line = line.rstrip()
    if line.startswith("From:"):
        print(line)

1
The for loop processes each file line one at a time.
2
rstrip() removes trailing whitespace and newline characters from the current line.
3
startswith("From:") checks whether the cleaned line begins with From:.
4
print(line) displays only matching lines without the extra spacing caused by the original newline.

Output

Only cleaned lines beginning with From: are displayed.

How does each line move through prefix checking, trailing-whitespace removal, and display?

How does each input line branch depending on whether startswith() returns True or False?

Common Mistakes

Common mistake
Treating startswith() as a general search method
What it looks like
```
"Message from Sam".startswith("From:")
```
Why it fails
The requested text is not at the beginning of the string, so startswith() returns False even though related text appears later.
Do this instead
Use startswith() when the position at the beginning matters; do not expect it to find a substring anywhere.
Common mistake
Printing file lines directly
What it looks like
```
for line in file:
    print(line)
```
Why it fails
Each file line already includes a trailing newline, and print() adds another newline.
Do this instead
Call rstrip() on the file line before printing it.
Common mistake
Removing only one final character manually
Why it fails
A line can contain multiple trailing whitespace characters, including spaces, tabs, or newlines.
Do this instead
Use rstrip(), which removes trailing whitespace and newline characters together.
Common mistake
Checking every line without filtering
What it looks like
```
for line in file:
    line = line.rstrip()
    print(line)
```
Why it fails
The code cleans the output but displays lines with every prefix.
Do this instead
Add a startswith() condition when only a particular type of line is needed.

Try the Workflow

EASY

Write a loop that reads lines from a file, removes trailing whitespace, and prints only lines beginning with "ERROR:".

Hints

Process the file one line at a time with a for loop.
Assign the result of line.rstrip() back to line.
Use if line.startswith("ERROR:") before print(line).

Your attempt (not saved)

python

for line in file:
    line = line.rstrip()
    if line.startswith("ERROR:"):
        print(line)

for line in file:
    line = line.rstrip()
    if line.startswith("ERROR:"):
        print(line)

Output (expected)

Only cleaned lines beginning with ERROR: are displayed.

Key Takeaways

1
startswith() checks only whether a string begins with a specified prefix and returns True or False.
2
File lines include trailing newline characters, so printing them directly can create double spacing.
3
rstrip() removes trailing whitespace and newline characters from the right end of a string.
4
A practical filtering pattern is to call rstrip(), test startswith(), and print only matching lines.
5
This pattern is useful for structured text such as email files and log files, where prefixes identify different kinds of lines.

At a glance

Use startswith() for prefix matching, not for searching anywhere within a string.
A file line may already contain a newline before print() adds its own.
Use rstrip() to remove trailing whitespace and prevent double-spaced output.
Combine rstrip() and startswith() in a loop to cleanly filter and display selected lines.

Related concepts

More concepts starting with H

The Hidden Newline

Checking a Prefix

Removing Trailing Characters

Filtering File Lines

Common Mistakes

Try the Workflow

Key Takeaways

At a glance

Related Topics

Related concepts