How to Get the Length of a String in
Python Stack Overflow
Table of Contents
- Introduction
- Using len()
- Why len() Returns the Length
- Checking String Length With
if Statements
- Using For Loops to Iterate
Over Strings
- Getting the Length of
Strings in Lists and Dictionaries
- Common Errors and Edge Cases
- Managing Different String
Types
- Length vs Size in Python
- Checking Length Before
Manipulating Strings
- When String Length Matters
for Performance
- Length of Multiline Strings
- Handling Leading and
Trailing Whitespace
- Encoding Considerations
- Checking for Empty Strings
- Splitting Strings First vs
Getting Length
- Alternatives to len()
- Getting Length in Python 2
vs Python 3
- Length of Strings vs Length
of Containers
- Use Cases for String Length
- Length Limitations
- Built-in len()
Implementation
- Length Caching and
Optimization
- Length of Collections with
Strings
- Strings vs Byte Strings
- Logging and Debugging String
Lengths
- Length in Web
Development
- Conclusion
- FAQs
- What is the fastest way to get string length in Python?
- Does len() work on all iterables in Python?
- What happens if I pass a number to len() instead of a string?
- Is there a max string length limit in Python?
- How can I get the length of each string in a list?
- What is the difference between len() and size() in Python?
- Will len() count Unicode characters properly?
- Can I get the length of a Python string object directly?
- Does len() still work if my string contains newlines or tabs?
- How do I check if a string meets a minimum length requirement?
Introduction
Determining the number of
characters in a string is a frequent requirement in Python programming. The
built-in `len()` function provides the most convenient way to get the length of
a string in Python.
Understanding how to properly use
`len()` for strings is an essential skill for Python developers. This in-depth
guide covers all aspects of getting, using, and manipulating string lengths in
Python code.
We will dig into how `len()`
works under the hood, best practices and warnings when working with string
lengths, use cases ranging from string manipulation to performance
optimizations, differences across Python versions, and much more. Read on to
master the many nuances of string length in Python!
Using len()
The easiest and most common
way to get the length of a string in Python is to call the `len()` built-in
function, passing the string as a parameter:
```python
my_string = "Hello
world"
string_length = len(my_string) #
11
```
`len()` will return the
count of characters in the string. This works for any string, including
literals, variables, function arguments, user input, etc:
```python
print(len("Python")) #
6
my_str = "Programming
language"
print(len(my_str)) # 18
def get_length(text):
return len(text)
print(get_length("Java"))
# 4
```
Some key properties of
`len()`:
- It is very fast and runs in
constant time O(1), not actually iterating the string.
- Returns the length as an
integer.
- Excludes newlines and whitespace
padding.
- Counts Unicode characters
properly.
- Can be called directly on a
string or passed the string.
- Works on all Python string
types like str, bytes, and bytearray.
Because of its speed and
simplicity, `len()` is ideal for most string length operations in Python.
Why len() Returns the Length
Behind the scenes, Python stores
strings similarly to arrays or lists as a sequence of characters in order. This
allows fast lookup of any character by index.
It also enables the highly
optimized `len()` function. When you create a string in Python, whether via a
literal or concatenating strings, the length is automatically tracked and
stored.
So when `len(my_string)` is
called, Python simply returns the pre-calculated length property of that
string. It does not need to iterate through and count each character on every
call.
This is why `len()` is so much
faster than trying to calculate the length manually with a loop. It leverages
the internal sequence storage of strings.
Checking String Length With if Statements
One very common way to
utilize string lengths in Python is comparing them to expected values using
conditionals like `if` statements:
```python
user_name = input("Enter
your name: ")
if len(user_name) > 20:
print("Name must be 20 characters or less")
else:
print("Name input validated")
# Prints error if over 20 chars
entered
```
We can also use `if`
statements to compare the relative length of two strings:
```python
password1 = "123"
password2 = "Df3$2"
if len(password1) <
len(password2):
print("Password 2 meets min length requirement")
```
This makes it very easy to
validate data or implement logic that depends on string length comparisons.
Using For Loops to Iterate Over Strings
In addition to getting the
overall length, `len()` can be used with `for` loops to iterate through each
index of a string:
```python
my_string = "Python"
for i in range(len(my_string)):
print(my_string[i])
# Prints P y t h o n
```
Compared to iterating directly
over the string, this idiom allows you to also use the index `i` within the
loop body, like accessing individual characters.
You can also loop
backwards:
```python
for i in range(len(my_string)-1,
-1, -1):
print(my_string[i])
# Prints n o h t y P
```
So `len()` provides flexibility
when iterating over strings.
## Getting the Length of Strings
in Lists and Dictionaries
For sequences and mappings
that contain strings like lists and dictionaries, `len()` can be used to easily
get the length of each string element:
```python
languages = ["Python",
"Java", "JavaScript"]
for lang in languages:
print(len(lang))
# 6
# 4
# 10
```
And the same for dictionary
keys and values:
```python
user_data = {
"alice123": "Alice Smith",
"bob7": "Bob Lee",
"charlie": "Charlie Park"
}
for username, fullname in
user_data.items():
print(len(username))
# 8
# 4
# 7
```
This makes working with
collections of strings very concise.
Common Errors and Edge Cases
While `len()` generally
works great for getting string lengths, there are some potential pitfalls and
edge cases to be aware of:
- Calling `len()` on a variable
before it has a value will raise a TypeError. Initialize it first.
- Passing a number or other data
type to `len()` will also error. Convert it to a string first.
- Attempting to use negative
indexes on a string will crash. Use `my_string[len(my_string) - 1]` instead.
- Multi-byte Unicode characters
count as 1 each toward length.
- Leading/trailing whitespace is
included in length unless stripped first.
- Empty strings have a length 0.
Check forEmptyString before other string ops.
Watch out for these issues when
using `len()`!
Managing Different String Types
Python has a few different
string types including str, bytes, and bytearray. `len()` can be used to get
the length of all these string types:
```python
s = "Hello" # str
b = b"Hello" # bytes
ba = bytearray("Hello",
'utf-8') # bytearray
print(len(s)) # 5
print(len(b)) # 5
print(len(ba)) # 5
```
One caution - for bytes and
bytearray, the length reflects the raw bytes stored rather than Unicode
characters like in a str. So keep that distinction in mind.
Length vs Size in Python
In Python there is a difference
between the `len()` function and `__size__()` method:
- `len()` returns the count of
elements of a container like the number of chars in a string.
- `__size__()` returns the actual
byte size of an object in memory.
In most cases, you will want `len()`
rather than `__size__()` when working with strings and collections in Python.
The element count is usually more relevant than the raw byte size.
Checking Length Before Manipulating Strings
It's good practice to check
the length of a string before manipulating it or accessing indexes to avoid
errors:
```python
max_length = 50
user_input = input("Enter
your bio: ")
if len(user_input) >
max_length:
print("Bio must be under 50 chars")
elif len(user_input) == 0:
print("Bio cannot be empty!")
else:
# Safely index string here knowing 0 <= idx < len(user_input)
print(user_input[0])
```
We can also truncate long
strings to a max length:
```python
long_string = "This is a
really really really really long string"
if len(long_string) > 20:
long_string = long_string[:20]
print(long_string)
# This is a really
```
Defensively checking the length
first prevents crashes!
## When String Length Matters for
Performance
In general, calling `len()`
repeatedly on strings even in tight loops results in good performance.
But there are cases where you may
want to optimize string length operations for peak performance:
- Inside inner loops or
algorithms that call `len()` millions of times.
- On extremely long strings.
- On hardware with slow function
calls like microcontrollers.
For these cases, cache the
string length rather than re-calculating it constantly:
```python
my_string = "Hello
World"
# Store length
str_len = len(my_string)
for i in range(str_len):
print(my_string[i])
if str_len > 10:
print("Long string")
```
This pattern can speed up
algorithms with many `len()` calls.
Length of Multiline Strings
One interesting note about
`len()` is that it will count all characters including newlines in multiline
strings:
```python
multiline =
"""This is a
multiline
string with
newlines"""
print(len(multiline)) # 52
```
The newlines `\n` are included
like any other character. Use `rstrip()`, `lstrip()`, or `strip()` to remove
leading/trailing whitespace before getting the length if needed.
Handling Leading and Trailing Whitespace
Since `len()` includes
whitespace padding in the total length, you may want to standardize strings by
stripping whitespace before checking length:
```python
user_input = " abc
"
print(len(user_input)) # 9
stripped = user_input.strip()
print(len(stripped)) # 3
```
This can simplify length checks
and standardize string processing.
Encoding Considerations
An important note is
`len()` will operate on the encoded string characters. This becomes relevant if
dealing with multi-byte encodings like UTF-8:
```python
# UTF-8 encoding means é is 2
bytes
name = "José"
print(len(name)) # 4
```
So always keep the encoding in
mind if calculating expected string lengths.
Checking for Empty Strings
One common check is looking
for empty strings before further processing:
```python
input_str = input("Enter
something: ")
if len(input_str) == 0:
print("You didn't enter anything!")
else:
print("Input was: " + input_str)
```
This avoids indexing errors if
input is empty. An alternative is using `if not input_str:` which also catches
empty strings.
## Splitting Strings First vs Getting Length
A pattern you may see is
splitting a string before getting the length:
```python
# Option 1
split_string = my_string.split()
string_length = len(split_string)
# Option 2
string_length = len(my_string)
```
In most cases option 2 is
preferable for simplicity unless you specifically need the split output.
Alternatives to len()
While `len()` is ideal in
most cases, there are some alternatives:
- Convert string to list of chars
first via `list(my_string)` then check `len()` on that list.
- Loop through and increment a
counter for each char.
- Use regex with a pattern like
`r'.'` to count matches.
- For large strings, estimate
length using sampling rather than full `len()`.
But `len()` will be faster and
simpler in the majority of cases.
Getting Length in Python 2 vs Python 3
The `len()` function works the
same for strings in both Python 2 and 3. No changes needed there.
One relevant difference - Python
2 has ` basestring` as the base string type, while Python 3 just has `str`. But
`len()` works on both.
Length of Strings vs Length of Containers
Note that `len()` can be applied
to strings like `"abc"` but also to any container like lists and
dicts to get the number of elements.
So `len()` is broadly useful for
many data types in Python, not just strings.
Use Cases for String Length
Some examples of where
checking string length is useful:
- Validating form input like
usernames or passwords meet length requirements.
- Truncating strings to be within
size limits before processing.
- Breaking up long strings that
exceed size limits.
- Getting indexes for string
slicing and substring operations.
- Looping through each character
of a string iteratively.
- Padding and aligning strings to
specific lengths.
- Checking for empty or missing
input strings.
String length comes up in nearly
all string manipulation tasks!
Length Limitations
While Python strings can
theoretically be arbitrarily long, there are some practical limits to be aware
of:
- Python versions before 3.6: max
string size was 232 chars due to internals.
- Memory constraints: strings
over 1GB may cause memory errors.
- File I/O constraints: writing
very large strings to files.
- Network I/O constraints:
sending massive strings.
For most purposes strings under
2GB should be safe, but keep these potential limits in mind!
Built-in len() Implementation
For those curious, here is
simplified Python pseudocode showing what `len()` does under the hood:
```python
def len(string):
# Strings have a cached length property
return string.length
```
It simply returns the
pre-computed length property rather than actually counting characters each
time.
Length Caching and Optimization
Building on above - Python
aggressively caches string lengths. Some implementations even cache the lengths
of all literals like `"abc"` at compile time for max performance.
So in practice you should not see
much overhead even when calling `len()` in very hot code paths.
Length of Collections with Strings
Because `len()` works on
any container type, it's useful for getting length of collections containing
strings:
```python
names = ["Alice",
"Bob", "Charlie"]
lengths = [len(n) for n in names]
# [5, 3, 7]
max_length = len(max(names)) # 7
```
This provides flexibility in
calculating lengths across collections of strings.
Strings vs Byte Strings
One nuance is that `len()` will
operate on raw bytes for bytestring types like `b"abc"`, while it
counts Unicode characters for `"abc"`.
This can matter if you expect
string lengths in bytes rather than glyphs.
Logging and Debugging String Lengths
Logging and printing the
string length during debugging can help ensure your code is processing the
expected string values:
```python
user_input = input("Enter
your name: ")
print("DEBUG: Input length
is: ", len(user_input))
# Do validation, processing, etc
```
Adding len information to logs is
handy for troubleshooting.
Length in Web Development
On the web, `len()` is
useful for handling request and response bodies:
```python
# Flask example
@app.route("/search")
def search():
query = request.args.get("query")
if not query or len(query) < 3:
return "Query too short"
# Do query lookup...
```
And can help ensure response
sizes stay within bounds too.
Conclusion
The built-in `len()` function
enables easy access to string lengths in Python. It should be your default tool
any time you need to know how many characters are in a string.
Common use cases range from input
validation to string slicing to performance optimizations. Knowing the ins and
outs of `len()` helps write better Python code.
Some key takeaways:
- `len()` runs in O(1) time,
optimized for speed.
- Works on any Python string type
like str, bytes, etc.
- Counts Unicode glyphs properly,
watch for multi-byte chars.
- Excludes newlines and padding
whitespace.
- Can leverage string length for
slicing, iteration, conditional checks, and more.
- Avoid common pitfalls like
empty strings or unset variables.
- Cache length if needed for
performance in hot loops.
Remember these best practices and
techniques next time string length is needed in your Python code!
FAQs
What is the fastest way to get string length in Python?
The built-in `len()` function is
optimized to very quickly return the pre-calculated length of a string in
constant time O(1). It is significantly faster than writing your own length
calculation.
Does len() work on all iterables in Python?
Yes, `len()` can be called on any
object in Python that implements the `__len__()` magic method, including
strings, lists, tuples, dictionaries, sets, and custom classes that implement
`__len__()`. It allows getting the length of any iterable.
What happens if I pass a number to len() instead of a string?
Passing a number like `len(5)`
will result in a TypeError, as numbers do not implement `__len__()` in Python.
You would first need to convert it to a string with `str()` before passing to
`len()`.
Is there a max string length limit in Python?
No, Python strings can be
arbitrarily large in theory. However, there may be practical limits based on
the amount of memory available. Very large strings over 1GB may cause
MemoryErrors. But for most purposes, strings can safely contain millions of
characters.
How can I get the length of each string in a list?
To get the individual
lengths for a list of strings, use a list comprehension:
```python
lengths = [len(s) for s in
list_of_strings]
```
Or with map():
```python
lengths = map(len,
list_of_strings)
```
Both options will return a new
list containing the length of each string.
What is the difference between len() and size() in Python?
`len()` returns the number of
items for a container like a string or list. `size()` returns the actual byte
size that an object takes up in memory. `len()` is usually more useful for
counting elements.
Will len() count Unicode characters properly?
Yes, `len()` counts the number of
Unicode code points or glyphs in a string, regardless of if they are 1, 2, 3 or
4 bytes per character. It handles Unicode strings correctly.
Can I get the length of a Python string object directly?
Yes, you can call
`my_string.__len__()` and it will return the same result as `len(my_string)`.
But calling `len()` directly is preferred for clarity and consistency.
### Does len() still work if my
string contains newlines or tabs?
Yes, `len()` returns the total
number of characters including newlines, tabs, and any other whitespace
characters. Use `strip()` to remove whitespace before getting length if needed.
How do I check if a string meets a minimum length requirement?
You can check with:
```python
if len(my_string) >= 10:
print("String meets minimum length")
```
This validates the length meets
the required threshold.