Python: Dictionary Methods

By Xah Lee. Date: . Last updated: .

This page is a complete list of Python dictionary methods.

For a basic tutorial, see: Python: Dictionary

Length

length
syntaxmeaning
len(d)return number of items in d. For example, len({"a":8,"b":9})⇒ 2

Get, Set, Delete

get and set and delete
syntaxmeaning
d[k]return the value of key k if exist, else KeyError.
d.get(k)return the value of key k if exist, else return None.
d.get(k,v)return the value of key k if exist, else return v.
d[k] = vSet a value v.
del d[k]Remove the value of key k if exist, else KeyError.
# -*- coding: utf-8 -*-
# python

# example of ‹dictionary›.get(…)

hh = {"a":1, "b":2}

print hh.get("b")   # 2
print hh.get("x")   # None
print hh.get("x", 8)  # 8

Check Existence

test existence
syntaxmeaning
k in dreturn True if d has a key k, else False.
k not in d
d.has_key(k)return true if key k exist in d. This method is deprecated and removed in Python 3.

Get All Keys, Get All Keys Values

Get All Keys, Get All Keys Values
syntaxmeaning
d.keys()return a list that's all key's values. (each element is a new copy)
d.values()return a list that's all key's values. (each element is a new copy)
d.items()return a list of 2-tuples, each is (key, value). (each element is a new copy)
# -*- coding: utf-8 -*-
# python

# example of ‹dictionary›.keys() and ‹dictionary›.keys() and ‹dictionary›.items()

dd = {"a":1, "b":2, "c":3}

kk = dd.keys()
print kk    # ['a', 'c', 'b']

vv = dd.values()
print vv    # [1, 3, 2]

tt = dd.items()
print tt    # [('a', 1), ('c', 3), ('b', 2)]

print type(kk)  # <type 'list'>
print type(vv)  # <type 'list'>
print type(tt)  # <type 'list'>

Loop Thru Key/Value Pairs

Loop Thru Key/Valuese Pairs
syntaxmeaning
d.iterkeys()return a iterator. Each element is a key. A short syntax is iter(d)
d.itervalues()return a iterator. Each element is a key's value.
d.iteritems()return a iterator. Each element is (key, value) pair.
# -*- coding: utf-8 -*-
# python

# example of .iterkeys() and .iterkeys() and .iteritems()

dd = {"a":1, "b":2, "c":3}

ik = dd.iterkeys()
print ik # <dictionary-keyiterator object at 0x7f463d398f70>

iv = dd.itervalues()
print iv   # <dictionary-valueiterator object at 0x7f463d398fc8>

ii = dd.iteritems()
print ii    # <dictionary-itemiterator object at 0x7f463d32f050>

print type(ik)  # <type 'dictionary-keyiterator'>
print type(iv)  # <type 'dictionary-valueiterator'>
print type(ii)  # <type 'dictionary-itemiterator'>

Pop, Update

pop, update
syntaxmeaning
d.pop(k)Remove and return its value if key k exist, else KeyError.
d.pop(k,v)Remove and return its value if key k exist, else v.
d.popitem()Remove and return arbitrary (key, value) pair. If d is empty, KeyError.

Set Value, Update

set value, update
syntaxmeaning
d.setdefault(k)If key k exist, return its value, else, add key with a value of None.
d.setdefault(k, v)If key k exist, return its value, else, insert key with a value of v
d.update(v)Update the dictionary with the key/value pairs from v, overwriting existing keys. Return None. v can be a dictionary or iterable (list, tuple) where each element is iterable of length 2, or can be a key1=val1, key2=val2, …
# -*- coding: utf-8 -*-
# python

# example of ‹dict›.update(…)

aa = {"a":1}

aa.update( [ [3,4], ("a", 2) ] )
print aa    # {'a': 2, 3: 4}

aa.update([(5,6), (7,8)])
print aa    # {'a': 2, 3: 4, 5: 6, 7: 8}

aa.update(aa=8, bb=9)
print aa    # {'a': 2, 'aa': 8, 3: 4, 5: 6, 'bb': 9, 7: 8}

Clear, Copy

clear, copy
syntaxmeaning
d.clear()Remove all items.
d.copy()return a shallow copy of d. [see How to Copy a Nested List in Python?]
d.fromkeys(seq)return a new dictionary with keys from sequence seq (list or tuple). The values are all None.
d.fromkeys(seq, v)return a new dictionary with keys from sequence seq. The values are all ‹›.
# -*- coding: utf-8 -*-
# python

# example of ‹dictionary›.fromkeys(…)

hh = {"a":1, "b":2, "c":3}

h2 = hh.fromkeys([8,9,10])
print h2    # {8: None, 9: None, 10: None}

h3 = hh.fromkeys([8,9,10],"x")
print h3    # {8: 'x', 9: 'x', 10: 'x'}

print hh    # {'a': 1, 'c': 3, 'b': 2}

Dictionary View

view
syntaxmeaning
d.viewitems()a view to a kill 😨
d.viewkeys()a view to a kill 😨
d.viewvalues()a view to a kill 😨

Change Lists into Dictionary

Example of using zip(…). It does transposition.

# -*- coding: utf-8 -*-
# python

# convert 2 lists into a dictionary, use zip()

aa = [1, 2, 3]
bb = ["a", "b", "c"]

print dict(zip(aa, bb))
# ⇒ {1: 'a', 2: 'b', 3: 'c'}
# -*- coding: utf-8 -*-
# python

# example of using zip(). It does transposition.

aa = [1, 2, 3, 4]
bb = ["a", "b", "c", "d"]
cc = [10, 20, 30, 40]
print zip(aa, bb, cc)
# ⇒ [(1, 'a', 10), (2, 'b', 20), (3, 'c', 30), (4, 'd', 40)]

Python: Dictionary Topic

  1. Dictionary
  2. Loop Thru Dictionary
  3. Dictionary Methods

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Python by Example

  1. Python Basics
  2. Print Version String
  3. Builtin Help
  4. Quote String
  5. String Operations
  6. String Methods
  7. Format String
  8. True, False
  9. if then else
  10. for, while, Loops
  11. List Basics
  12. Loop Thru List
  13. Map Function to List
  14. List Comprehension
  15. List Methods
  16. Dictionary
  17. Loop Thru Dict
  18. Dict Methods
  19. Function
  20. Class
  21. List Modules
  22. Write a Module
  23. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. Manipulate Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency

Web

  1. Send Email
  2. GET Web Page
  3. Web Crawler
  4. HTTP POST
  5. Check Page Load Size
  6. Thumbnail Generation

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Complex Numbers

Advanced

  1. Sort
  2. Copy Nested List
  3. Tuple vs List
  4. Sets, Union, Intersection
  5. Closure in Python 2
  6. Decorator
  7. Append String in Loop
  8. Timing f timeit
  9. Keyword Arg Default Value Unstable