Len

In Python programs we often need to get the size or count of elements. In strings we need a length. The len() built-in helps here.

Len details

Often we can optimize performance with len. The number of elements is stored on the object, not calculated, so len is fast.

Strings

Len returns the number of characters in a string. It counts spaces, punctuation, all characters the same. We must be careful with taking the len of a None variable—this fails.

Next The second len call tests an empty string. This string has zero characters but is not None.

Tip Len relies on the type of the variable passed to it. A NoneType has no len built-in support.

# Has length of 3:
value = "cat"
print(len(value))

# Has length of 0:
value = ""
print(len(value))

# Causes TypeError:
value = None
print(len(value))3
0
Traceback (most recent call last):
  File "C:\programs\file.py", line 13, in <module>
    print(len(value))
TypeError: object of type 'NoneType' has no len()

`List`, dictionary

Len() returns the number of elements in a collection. For a collection with nested, sub-collections, counting is shallow: not all nested elements are considered.

Info For the dictionary, each pair is counted as one unit. Keys and values are not independent.

# Get length of list with len.
elements = [1, 2, 3]
print(len(elements))

# Get length of tuple.
items = ("cat", "dog", "bird", "shark")
print(len(items))

# Length of example set (key count).
set = {100, 200, 300}
print(len(set))

# Length of dictionary (pair count).
lookup = {"cat" : 4, "centipede" : 100}
print(len(lookup))3
4
3
2

Nested lists

Let us revisit nested collections. A collection itself is an element, so it counts just one time. The len built-in does not recurse—it is simple and does not even loop.

# A nested list:
list = [1, 2, [4, 5]]

# Shallow count of elements.
print(len(list))
print(len(list[2]))3
2

Error

We cannot just take the len of any variable. This program attempts to take the length of an int variable. And it fails, miserably, with a TypeError that ends its operation.

Note Conceptually len() counts countable units: chars in a string, elements in a list. A number has digits, but no other "units."

value = 100

# Cannot take length of int:
length = len(value)Traceback (most recent call last):
  File "C:\programs\file.py", line 6, in <module>
    length = len(value)
TypeError: object of type 'int' has no len()

Benchmark

The len of collections and strings is stored as a number in memory. It is not computed, as in a loop, each time it is accessed. For this reason, len is much faster than a loop.

Version 1 In this version of the code, we access the length of a string with len in a loop. This is timed.

Version 2 We test a for-loop version. The end result is the same (each character is counted).

Result It is many times faster to access len. The for-loop is useful only when counting chars, where their values matter.

import time

value = "characters"

print(time.time())

# Version 1: len
for i in range(0, 1000000):
    length = len(value)
    if length != 10:
        raise Exception()

print(time.time())

# Version 2: count chars
for i in range(0, 1000000):
    length = 0
    for c in value:
        length += 1
    if length != 10:
        raise Exception()

print(time.time())1406752804.325871
1406752804.606887    len      = 0.281 s
1406752806.05097     for-loop = 1.444 s

Summary

A length cannot be negative. So we can use len as a loop boundary: this is a convenient way to loop over a list. But when not needed, avoiding len is ideal.

Loop suggestion

Consider a for-in loop to avoid using len—this loop construct will enumerate each element in a collection, and no indexes are needed.

Len