Python Programming/Dictionaries

A dictionary in Python is a collection of unordered values accessed by key rather than by index. The keys have to be hashable: integers, floating point numbers, strings, tuples, and, frozensets are hashable. Lists, dictionaries, and sets other than frozensets are not hashable. Dictionaries were available as early as in Python 1.4.

Overview

Dictionaries in Python at a glance:

dict1={}# Create an empty dictionarydict2=dict()# Create an empty dictionary 2dict2={"r":34,"i":56}# Initialize to non-empty valuedict3=dict([("r",34),("i",56)])# Init from a list of tuplesdict4=dict(r=34,i=56)# Initialize to non-empty value 3dict1["temperature"]=32# Assign value to a keyif"temperature"indict1:# Membership test of a key AKA key existsdeldict1["temperature"]# Delete AKA removeequalbyvalue=dict2==dict3itemcount2=len(dict2)# Length AKA size AKA item countisempty2=len(dict2)==0# Emptiness testforkeyindict2:# Iterate via keysprint(key,dict2[key])# Print key and the associated valuedict2[key]+=10# Modify-access to the key-value pairforkeyinsorted(dict2):# Iterate via keys in sorted order of the keysprint(key,dict2[key])# Print key and the associated valueforvalueindict2.values():# Iterate via valuesprint(value)forkey,valueindict2.items():# Iterate via pairsprint(key,value)dict5={}# {x: dict2[x] + 1 for x in dict2 } # Dictionary comprehension in Python 2.7 or laterdict6=dict2.copy()# A shallow copydict6.update({"i":60,"j":30})# Add or overwrite; a bit like list's extenddict7=dict2.copy()dict7.clear()# Clear AKA empty AKA erasesixty=dict6.pop("i")# Remove key i, returning its valueprint(dict1,dict2,dict3,dict4,dict5,dict6,dict7,equalbyvalue,itemcount2,sixty)

Dictionary notation

Dictionaries may be created directly or converted from sequences. Dictionaries are enclosed in curly braces, {}

>>>d={'city':'Paris','age':38,(102,1650,1601):'A matrix coordinate'}>>>seq=[('city','Paris'),('age',38),((102,1650,1601),'A matrix coordinate')]>>>d{'city':'Paris','age':38,(102,1650,1601):'A matrix coordinate'}>>>dict(seq){'city':'Paris','age':38,(102,1650,1601):'A matrix coordinate'}>>>d==dict(seq)True

Also, dictionaries can be easily created by zipping two sequences.

>>>seq1=('a','b','c','d')>>>seq2=[1,2,3,4]>>>d=dict(zip(seq1,seq2))>>>d{'a':1,'c':3,'b':2,'d':4}

Operations on Dictionaries

The operations on dictionaries are somewhat unique. Slicing is not supported, since the items have no intrinsic order.

>>>d={'a':1,'b':2,'cat':'Fluffers'}>>>d.keys()['a','b','cat']>>>d.values()[1,2,'Fluffers']>>>d['a']1>>>d['cat']='Mr. Whiskers'>>>d['cat']'Mr. Whiskers'>>>'cat'indTrue>>>'dog'indFalse

Combining two Dictionaries

You can combine two dictionaries by using the update method of the primary dictionary. Note that the update method will merge existing elements if they conflict.

>>>d={'apples':1,'oranges':3,'pears':2}>>>ud={'pears':4,'grapes':5,'lemons':6}>>>d.update(ud)>>>d{'grapes':5,'pears':4,'lemons':6,'apples':1,'oranges':3}>>>

Deleting from dictionary

deldictionaryName[membername]

Exercises

Write a program that:

1. Asks the user for a string, then creates the following dictionary. The values are the letters in the string, with the corresponding key being the place in the string. https://docs.python.org/2/tutorial/datastructures.html#looping-techniques
2. Replaces the entry whose key is the integer 3, with the value "Pie".
3. Asks the user for a string of digits, then prints out the values corresponding to those digits.

External links

• 5.5. Dictionaries in Tutorial, docs.python.org
• 5.8. Mapping Types in Library Doc, docs.python.org