Remove an item from a dictionary when its key is unknown - python

What is the best way to remove an item from a dictionary by value, i.e. when the item's key is unknown? Here's a simple approach:
for key, item in some_dict.items():
if item is item_to_remove:
del some_dict[key]
Are there better ways? Is there anything wrong with mutating (deleting items) from the dictionary while iterating it?


The dict.pop(key[, default]) method allows you to remove items when you know the key. It returns the value at the key if it removes the item otherwise it returns what is passed as default. See the docs.'
Example:
>>> dic = {'a':1, 'b':2}
>>> dic
{'a': 1, 'b': 2}
>>> dic.pop('c', 0)
0
>>> dic.pop('a', 0)
1
>>> dic
{'b': 2}


Be aware that you're currently testing for object identity (is only returns True if both operands are represented by the same object in memory - this is not always the case with two object that compare equal with ==). If you are doing this on purpose, then you could rewrite your code as
some_dict = {key: value for key, value in some_dict.items()
if value is not value_to_remove}
But this may not do what you want:
>>> some_dict = {1: "Hello", 2: "Goodbye", 3: "You say yes", 4: "I say no"}
>>> value_to_remove = "You say yes"
>>> some_dict = {key: value for key, value in some_dict.items() if value is not value_to_remove}
>>> some_dict
{1: 'Hello', 2: 'Goodbye', 3: 'You say yes', 4: 'I say no'}
>>> some_dict = {key: value for key, value in some_dict.items() if value != value_to_remove}
>>> some_dict
{1: 'Hello', 2: 'Goodbye', 4: 'I say no'}
So you probably want != instead of is not.


a = {'name': 'your_name','class': 4}
if 'name' in a: del a['name']


A simple comparison between del and pop():
import timeit
code = """
results = {'A': 1, 'B': 2, 'C': 3}
del results['A']
del results['B']
"""
print timeit.timeit(code, number=100000)
code = """
results = {'A': 1, 'B': 2, 'C': 3}
results.pop('A')
results.pop('B')
"""
print timeit.timeit(code, number=100000)
result:
0.0329667857143
0.0451040902256
So, del is faster than pop().


I'd build a list of keys that need removing, then remove them. It's simple, efficient and avoids any problem about simultaneously iterating over and mutating the dict.
keys_to_remove = [key for key, value in some_dict.iteritems()
if value == value_to_remove]
for key in keys_to_remove:
del some_dict[key]


items() returns a list, and it is that list you are iterating, so mutating the dict in the loop doesn't matter here. If you were using iteritems() instead, mutating the dict in the loop would be problematic, and likewise for viewitems() in Python 2.7.
I can't think of a better way to remove items from a dict by value.


c is the new dictionary, and a is your original dictionary, {'z','w'}
are the keys you want to remove from a
c = {key:a[key] for key in a.keys() - {'z', 'w'}}
Also check: https://www.safaribooksonline.com/library/view/python-cookbook-3rd/9781449357337/ch01.html


y={'username':'admin','machine':['a','b','c']}
if 'c' in y['machine'] : del y['machine'][y['machine'].index('c')]


There is nothing wrong with deleting items from the dictionary while iterating, as you've proposed. Be careful about multiple threads using the same dictionary at the same time, which may result in a KeyError or other problems.
Of course, see the docs at http://docs.python.org/library/stdtypes.html#typesmapping


This is how I would do it.
for key in some_dict.keys():
if some_dict[key] == item_to_remove:
some_dict.pop(key)
break

Related

Can't acces dictionary value by itteration on expression generated dictionary [duplicate]

d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print(key, 'corresponds to', d[key])
How does Python recognize that it needs only to read the key from the dictionary? Is key a special keyword, or is it simply a variable?

key is just a variable name.
for key in d:
will simply loop over the keys in the dictionary, rather than the keys and values. To loop over both key and value you can use the following:
For Python 3.x:
for key, value in d.items():
For Python 2.x:
for key, value in d.iteritems():
To test for yourself, change the word key to poop.
In Python 3.x, iteritems() was replaced with simply items(), which returns a set-like view backed by the dict, like iteritems() but even better.
This is also available in 2.7 as viewitems().
The operation items() will work for both 2 and 3, but in 2 it will return a list of the dictionary's (key, value) pairs, which will not reflect changes to the dict that happen after the items() call. If you want the 2.x behavior in 3.x, you can call list(d.items()).

It's not that key is a special word, but that dictionaries implement the iterator protocol. You could do this in your class, e.g. see this question for how to build class iterators.
In the case of dictionaries, it's implemented at the C level. The details are available in PEP 234. In particular, the section titled "Dictionary Iterators":
Dictionaries implement a tp_iter slot that returns an efficient
iterator that iterates over the keys of the dictionary. [...] This
means that we can write
for k in dict: ...
which is equivalent to, but much faster than
for k in dict.keys(): ...
as long as the restriction on modifications to the dictionary
(either by the loop or by another thread) are not violated.
Add methods to dictionaries that return different kinds of
iterators explicitly:
for key in dict.iterkeys(): ...
for value in dict.itervalues(): ...
for key, value in dict.iteritems(): ...
This means that for x in dict is shorthand for for x in
dict.iterkeys().
In Python 3, dict.iterkeys(), dict.itervalues() and dict.iteritems() are no longer supported. Use dict.keys(), dict.values() and dict.items() instead.

Iterating over a dict iterates through its keys in no particular order, as you can see here:
(This is no longer the case in Python 3.6, but note that it's not guaranteed behaviour yet.)
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> list(d)
['y', 'x', 'z']
>>> d.keys()
['y', 'x', 'z']
For your example, it is a better idea to use dict.items():
>>> d.items()
[('y', 2), ('x', 1), ('z', 3)]
This gives you a list of tuples. When you loop over them like this, each tuple is unpacked into k and v automatically:
for k,v in d.items():
print(k, 'corresponds to', v)
Using k and v as variable names when looping over a dict is quite common if the body of the loop is only a few lines. For more complicated loops it may be a good idea to use more descriptive names:
for letter, number in d.items():
print(letter, 'corresponds to', number)
It's a good idea to get into the habit of using format strings:
for letter, number in d.items():
print('{0} corresponds to {1}'.format(letter, number))

key is simply a variable.
For Python2.X:
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> for my_var in d:
>>> print my_var, 'corresponds to', d[my_var]
x corresponds to 1
y corresponds to 2
z corresponds to 3
... or better,
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.iteritems():
print the_key, 'corresponds to', the_value
For Python3.X:
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.items():
print(the_key, 'corresponds to', the_value)

When you iterate through dictionaries using the for .. in ..-syntax, it always iterates over the keys (the values are accessible using dictionary[key]).
To iterate over key-value pairs, use the following:
for k,v in dict.iteritems() in Python 2
for k,v in dict.items() in Python 3

This is a very common looping idiom. in is an operator. For when to use for key in dict and when it must be for key in dict.keys() see David Goodger's Idiomatic Python article (archived copy).

I have a use case where I have to iterate through the dict to get the key, value pair, also the index indicating where I am. This is how I do it:
d = {'x': 1, 'y': 2, 'z': 3}
for i, (key, value) in enumerate(d.items()):
print(i, key, value)
Note that the parentheses around the key, value are important, without them, you'd get an ValueError "not enough values to unpack".

Iterating over dictionaries using 'for' loops
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
...
How does Python recognize that it needs only to read the key from the
dictionary? Is key a special word in Python? Or is it simply a
variable?
It's not just for loops. The important word here is "iterating".
A dictionary is a mapping of keys to values:
d = {'x': 1, 'y': 2, 'z': 3}
Any time we iterate over it, we iterate over the keys. The variable name key is only intended to be descriptive - and it is quite apt for the purpose.
This happens in a list comprehension:
>>> [k for k in d]
['x', 'y', 'z']
It happens when we pass the dictionary to list (or any other collection type object):
>>> list(d)
['x', 'y', 'z']
The way Python iterates is, in a context where it needs to, it calls the __iter__ method of the object (in this case the dictionary) which returns an iterator (in this case, a keyiterator object):
>>> d.__iter__()
<dict_keyiterator object at 0x7fb1747bee08>
We shouldn't use these special methods ourselves, instead, use the respective builtin function to call it, iter:
>>> key_iterator = iter(d)
>>> key_iterator
<dict_keyiterator object at 0x7fb172fa9188>
Iterators have a __next__ method - but we call it with the builtin function, next:
>>> next(key_iterator)
'x'
>>> next(key_iterator)
'y'
>>> next(key_iterator)
'z'
>>> next(key_iterator)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
When an iterator is exhausted, it raises StopIteration. This is how Python knows to exit a for loop, or a list comprehension, or a generator expression, or any other iterative context. Once an iterator raises StopIteration it will always raise it - if you want to iterate again, you need a new one.
>>> list(key_iterator)
[]
>>> new_key_iterator = iter(d)
>>> list(new_key_iterator)
['x', 'y', 'z']
Returning to dicts
We've seen dicts iterating in many contexts. What we've seen is that any time we iterate over a dict, we get the keys. Back to the original example:
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
If we change the variable name, we still get the keys. Let's try it:
>>> for each_key in d:
... print(each_key, '=>', d[each_key])
...
x => 1
y => 2
z => 3
If we want to iterate over the values, we need to use the .values method of dicts, or for both together, .items:
>>> list(d.values())
[1, 2, 3]
>>> list(d.items())
[('x', 1), ('y', 2), ('z', 3)]
In the example given, it would be more efficient to iterate over the items like this:
for a_key, corresponding_value in d.items():
print(a_key, corresponding_value)
But for academic purposes, the question's example is just fine.

For Iterating through dictionaries, The below code can be used.
dictionary= {1:"a", 2:"b", 3:"c"}
#To iterate over the keys
for key in dictionary.keys():
print(key)
#To Iterate over the values
for value in dictionary.values():
print(value)
#To Iterate both the keys and values
for key, value in dictionary.items():
print(key,'\t', value)

You can check the implementation of CPython's dicttype on GitHub. This is the signature of method that implements the dict iterator:
_PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey,
PyObject **pvalue, Py_hash_t *phash)
CPython dictobject.c

To iterate over keys, it is slower but better to use my_dict.keys(). If you tried to do something like this:
for key in my_dict:
my_dict[key+"-1"] = my_dict[key]-1
it would create a runtime error because you are changing the keys while the program is running. If you are absolutely set on reducing time, use the for key in my_dict way, but you have been warned.

If you are looking for a clear and visual example:
cat = {'name': 'Snowy', 'color': 'White' ,'age': 14}
for key , value in cat.items():
print(key, ': ', value)
Result:
name: Snowy
color: White
age: 14

This will print the output in sorted order by values in ascending order.
d = {'x': 3, 'y': 1, 'z': 2}
def by_value(item):
return item[1]
for key, value in sorted(d.items(), key=by_value):
print(key, '->', value)
Output:
y -> 1
z -> 2
x -> 3

Let's get straight to the point. If the word key is just a variable, as you have mentioned then the main thing to note is that when you run a 'FOR LOOP' over a dictionary it runs through only the 'keys' and ignores the 'values'.
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print (key, 'corresponds to', d[key])
rather try this:
d = {'x': 1, 'y': 2, 'z': 3}
for i in d:
print (i, 'corresponds to', d[i])
but if you use a function like:
d = {'x': 1, 'y': 2, 'z': 3}
print(d.keys())
in the above case 'keys' is just not a variable, its a function.

A dictionary in Python is a collection of key-value pairs. Each key is connected to a value, and you can use a key to access the value associated with that key. A key's value can be a number, a string, a list, or even another dictionary. In this case, threat each "key-value pair" as a separate row in the table: d is your table with two columns. the key is the first column, key[value] is your second column. Your for loop is a standard way to iterate over a table.

How to loop through dataframes in dictionary? Python [duplicate]

d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print(key, 'corresponds to', d[key])
How does Python recognize that it needs only to read the key from the dictionary? Is key a special keyword, or is it simply a variable?

key is just a variable name.
for key in d:
will simply loop over the keys in the dictionary, rather than the keys and values. To loop over both key and value you can use the following:
For Python 3.x:
for key, value in d.items():
For Python 2.x:
for key, value in d.iteritems():
To test for yourself, change the word key to poop.
In Python 3.x, iteritems() was replaced with simply items(), which returns a set-like view backed by the dict, like iteritems() but even better.
This is also available in 2.7 as viewitems().
The operation items() will work for both 2 and 3, but in 2 it will return a list of the dictionary's (key, value) pairs, which will not reflect changes to the dict that happen after the items() call. If you want the 2.x behavior in 3.x, you can call list(d.items()).

It's not that key is a special word, but that dictionaries implement the iterator protocol. You could do this in your class, e.g. see this question for how to build class iterators.
In the case of dictionaries, it's implemented at the C level. The details are available in PEP 234. In particular, the section titled "Dictionary Iterators":
Dictionaries implement a tp_iter slot that returns an efficient
iterator that iterates over the keys of the dictionary. [...] This
means that we can write
for k in dict: ...
which is equivalent to, but much faster than
for k in dict.keys(): ...
as long as the restriction on modifications to the dictionary
(either by the loop or by another thread) are not violated.
Add methods to dictionaries that return different kinds of
iterators explicitly:
for key in dict.iterkeys(): ...
for value in dict.itervalues(): ...
for key, value in dict.iteritems(): ...
This means that for x in dict is shorthand for for x in
dict.iterkeys().
In Python 3, dict.iterkeys(), dict.itervalues() and dict.iteritems() are no longer supported. Use dict.keys(), dict.values() and dict.items() instead.

Iterating over a dict iterates through its keys in no particular order, as you can see here:
(This is no longer the case in Python 3.6, but note that it's not guaranteed behaviour yet.)
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> list(d)
['y', 'x', 'z']
>>> d.keys()
['y', 'x', 'z']
For your example, it is a better idea to use dict.items():
>>> d.items()
[('y', 2), ('x', 1), ('z', 3)]
This gives you a list of tuples. When you loop over them like this, each tuple is unpacked into k and v automatically:
for k,v in d.items():
print(k, 'corresponds to', v)
Using k and v as variable names when looping over a dict is quite common if the body of the loop is only a few lines. For more complicated loops it may be a good idea to use more descriptive names:
for letter, number in d.items():
print(letter, 'corresponds to', number)
It's a good idea to get into the habit of using format strings:
for letter, number in d.items():
print('{0} corresponds to {1}'.format(letter, number))

key is simply a variable.
For Python2.X:
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> for my_var in d:
>>> print my_var, 'corresponds to', d[my_var]
x corresponds to 1
y corresponds to 2
z corresponds to 3
... or better,
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.iteritems():
print the_key, 'corresponds to', the_value
For Python3.X:
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.items():
print(the_key, 'corresponds to', the_value)

When you iterate through dictionaries using the for .. in ..-syntax, it always iterates over the keys (the values are accessible using dictionary[key]).
To iterate over key-value pairs, use the following:
for k,v in dict.iteritems() in Python 2
for k,v in dict.items() in Python 3

This is a very common looping idiom. in is an operator. For when to use for key in dict and when it must be for key in dict.keys() see David Goodger's Idiomatic Python article (archived copy).

I have a use case where I have to iterate through the dict to get the key, value pair, also the index indicating where I am. This is how I do it:
d = {'x': 1, 'y': 2, 'z': 3}
for i, (key, value) in enumerate(d.items()):
print(i, key, value)
Note that the parentheses around the key, value are important, without them, you'd get an ValueError "not enough values to unpack".

Iterating over dictionaries using 'for' loops
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
...
How does Python recognize that it needs only to read the key from the
dictionary? Is key a special word in Python? Or is it simply a
variable?
It's not just for loops. The important word here is "iterating".
A dictionary is a mapping of keys to values:
d = {'x': 1, 'y': 2, 'z': 3}
Any time we iterate over it, we iterate over the keys. The variable name key is only intended to be descriptive - and it is quite apt for the purpose.
This happens in a list comprehension:
>>> [k for k in d]
['x', 'y', 'z']
It happens when we pass the dictionary to list (or any other collection type object):
>>> list(d)
['x', 'y', 'z']
The way Python iterates is, in a context where it needs to, it calls the __iter__ method of the object (in this case the dictionary) which returns an iterator (in this case, a keyiterator object):
>>> d.__iter__()
<dict_keyiterator object at 0x7fb1747bee08>
We shouldn't use these special methods ourselves, instead, use the respective builtin function to call it, iter:
>>> key_iterator = iter(d)
>>> key_iterator
<dict_keyiterator object at 0x7fb172fa9188>
Iterators have a __next__ method - but we call it with the builtin function, next:
>>> next(key_iterator)
'x'
>>> next(key_iterator)
'y'
>>> next(key_iterator)
'z'
>>> next(key_iterator)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
When an iterator is exhausted, it raises StopIteration. This is how Python knows to exit a for loop, or a list comprehension, or a generator expression, or any other iterative context. Once an iterator raises StopIteration it will always raise it - if you want to iterate again, you need a new one.
>>> list(key_iterator)
[]
>>> new_key_iterator = iter(d)
>>> list(new_key_iterator)
['x', 'y', 'z']
Returning to dicts
We've seen dicts iterating in many contexts. What we've seen is that any time we iterate over a dict, we get the keys. Back to the original example:
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
If we change the variable name, we still get the keys. Let's try it:
>>> for each_key in d:
... print(each_key, '=>', d[each_key])
...
x => 1
y => 2
z => 3
If we want to iterate over the values, we need to use the .values method of dicts, or for both together, .items:
>>> list(d.values())
[1, 2, 3]
>>> list(d.items())
[('x', 1), ('y', 2), ('z', 3)]
In the example given, it would be more efficient to iterate over the items like this:
for a_key, corresponding_value in d.items():
print(a_key, corresponding_value)
But for academic purposes, the question's example is just fine.

For Iterating through dictionaries, The below code can be used.
dictionary= {1:"a", 2:"b", 3:"c"}
#To iterate over the keys
for key in dictionary.keys():
print(key)
#To Iterate over the values
for value in dictionary.values():
print(value)
#To Iterate both the keys and values
for key, value in dictionary.items():
print(key,'\t', value)

You can check the implementation of CPython's dicttype on GitHub. This is the signature of method that implements the dict iterator:
_PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey,
PyObject **pvalue, Py_hash_t *phash)
CPython dictobject.c

To iterate over keys, it is slower but better to use my_dict.keys(). If you tried to do something like this:
for key in my_dict:
my_dict[key+"-1"] = my_dict[key]-1
it would create a runtime error because you are changing the keys while the program is running. If you are absolutely set on reducing time, use the for key in my_dict way, but you have been warned.

If you are looking for a clear and visual example:
cat = {'name': 'Snowy', 'color': 'White' ,'age': 14}
for key , value in cat.items():
print(key, ': ', value)
Result:
name: Snowy
color: White
age: 14

This will print the output in sorted order by values in ascending order.
d = {'x': 3, 'y': 1, 'z': 2}
def by_value(item):
return item[1]
for key, value in sorted(d.items(), key=by_value):
print(key, '->', value)
Output:
y -> 1
z -> 2
x -> 3

Let's get straight to the point. If the word key is just a variable, as you have mentioned then the main thing to note is that when you run a 'FOR LOOP' over a dictionary it runs through only the 'keys' and ignores the 'values'.
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print (key, 'corresponds to', d[key])
rather try this:
d = {'x': 1, 'y': 2, 'z': 3}
for i in d:
print (i, 'corresponds to', d[i])
but if you use a function like:
d = {'x': 1, 'y': 2, 'z': 3}
print(d.keys())
in the above case 'keys' is just not a variable, its a function.

A dictionary in Python is a collection of key-value pairs. Each key is connected to a value, and you can use a key to access the value associated with that key. A key's value can be a number, a string, a list, or even another dictionary. In this case, threat each "key-value pair" as a separate row in the table: d is your table with two columns. the key is the first column, key[value] is your second column. Your for loop is a standard way to iterate over a table.

Python update a key in dict if it doesn't exist

I want to insert a key-value pair into dict if key not in dict.keys().
Basically I could do it with:
if key not in d.keys():
d[key] = value
But is there a better way? Or what's the pythonic solution to this problem?

You do not need to call d.keys(), so
if key not in d:
d[key] = value
is enough. There is no clearer, more readable method.
You could update again with dict.get(), which would return an existing value if the key is already present:
d[key] = d.get(key, value)
but I strongly recommend against this; this is code golfing, hindering maintenance and readability.

Use dict.setdefault():
>>> d = {'key1': 'one'}
>>> d.setdefault('key1', 'some-unused-value')
'one'
>>> d # d has not changed because the key already existed
{'key1': 'one'}
>>> d.setdefault('key2', 'two')
'two'
>>> d
{'key1': 'one', 'key2': 'two'}

Since Python 3.9 you can use the merge operator | to merge two dictionaries. The dict on the right takes precedence:
new_dict = old_dict | { key: val }
For example:
new_dict = { 'a': 1, 'b': 2 } | { 'b': 42 }
print(new_dict) # {'a': 1, 'b': 42}
Note: this creates a new dictionary with the updated values.

With the following you can insert multiple values and also have default values but you're creating a new dictionary.
d = {**{ key: value }, **default_values}
I've tested it with the most voted answer and on average this is faster as it can be seen in the following example, .
Speed test comparing a for loop based method with a dict comprehension with unpack operator method.
if no copy (d = default_vals.copy()) is made on the first case then the most voted answer would be faster once we reach orders of magnitude of 10**5 and greater. Memory footprint of both methods are the same.

You can also use this solution in only one line of code:
dict[dict_key] = dict.get(dict_key,value)
The second argument of dict.get is the value you want to assign to the key in case the key does not exist. Since this evaluates before the assignment to dict[dict_key] = , we can be sure that they key will exist when we try to access it.

Pick a key from a dictionary python

go through a dictionary picking keys from it in a loop?
For example lets say I have the following dictionary: {'hello':'world', 'hi':'there'}. Is there a way to for loop through the dictionary and print hello, hi?
on a similar note is there a way to say myDictionary.key[1] and that will return hi?

You can iterate over the keys of a dict with a for loop:
>>> for key in yourdict:
>>> print(key)
hi
hello
If you want them as a comma separated string you can use ', '.join.
>>> print(', '.join(yourdict))
hi, hello
on a similar note is there a way to say myDictionary.key1 and that will return hi
No. The keys in a dictionary are not in any particular order. The order that you see when you iterate over them may not be the same as the order you inserted them into the dictionary, and also the order could in theory change when you add or remove items.
if you need an ordered collection you might want to consider using another type such as a list, or an OrderedDict

You can use the .keys() method:
for key in myDictionary.keys():
print(key)
You can also use .items() to iterate through both at the same time:
for key, value in myDictionary.items():
print(key, value)

Using the dictionary name as a sequence produces all the keys:
>>> d={'hello':'world', 'hi':'there'}
>>> list(d)
['hi', 'hello']
so list({'hello':'world', 'hi':'there'})[1] produces element 1 of the list of keys.
This is of limited use, however, because dictionaries are unordered. And their order may be different than the order of insertion:
>>> d={'a': 'ahh', 'b': 'baa', 'c': 'coconut'}
>>> d
{'a': 'ahh', 'c': 'coconut', 'b': 'baa'}
You can do sorted(list({'hello':'world', 'hi':'there'}))[1] for the 1 element of a sorted list of the keys of your dict. That produces 'hi' in this case. Not the most readable or efficient though...
You should look at OrderedDict if you want a sorted order.
Or just sort into a list:
>>> d={'a': 'ahh', 'b': 'baa', 'c': 'coconut'}
>>> l=[(k,v) for k, v in d.items()]
>>> l.sort()
>>> l[1]
('b', 'baa')
>>> l[1][0]
'b'
You can reverse (k,v) to (v,k) if you want to sort by value instead of by key.

dict.iterkeys in Python 2, dict.keys in Python 3.
d = { 'hello': 'world', 'hi': 'there' }
for key in d.iterkeys():
print key

Sounds like a list of keys would meet your needs:
>>> d = { 'hello': 'world', 'hi': 'there' }
>>> keys = list(d)
>>> keys
['hi', 'hello']
>>> from random import choice
>>> choice(keys)
'hi'

Iterating over dictionaries using 'for' loops

d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print(key, 'corresponds to', d[key])
How does Python recognize that it needs only to read the key from the dictionary? Is key a special keyword, or is it simply a variable?

key is just a variable name.
for key in d:
will simply loop over the keys in the dictionary, rather than the keys and values. To loop over both key and value you can use the following:
For Python 3.x:
for key, value in d.items():
For Python 2.x:
for key, value in d.iteritems():
To test for yourself, change the word key to poop.
In Python 3.x, iteritems() was replaced with simply items(), which returns a set-like view backed by the dict, like iteritems() but even better.
This is also available in 2.7 as viewitems().
The operation items() will work for both 2 and 3, but in 2 it will return a list of the dictionary's (key, value) pairs, which will not reflect changes to the dict that happen after the items() call. If you want the 2.x behavior in 3.x, you can call list(d.items()).

It's not that key is a special word, but that dictionaries implement the iterator protocol. You could do this in your class, e.g. see this question for how to build class iterators.
In the case of dictionaries, it's implemented at the C level. The details are available in PEP 234. In particular, the section titled "Dictionary Iterators":
Dictionaries implement a tp_iter slot that returns an efficient
iterator that iterates over the keys of the dictionary. [...] This
means that we can write
for k in dict: ...
which is equivalent to, but much faster than
for k in dict.keys(): ...
as long as the restriction on modifications to the dictionary
(either by the loop or by another thread) are not violated.
Add methods to dictionaries that return different kinds of
iterators explicitly:
for key in dict.iterkeys(): ...
for value in dict.itervalues(): ...
for key, value in dict.iteritems(): ...
This means that for x in dict is shorthand for for x in
dict.iterkeys().
In Python 3, dict.iterkeys(), dict.itervalues() and dict.iteritems() are no longer supported. Use dict.keys(), dict.values() and dict.items() instead.

Iterating over a dict iterates through its keys in no particular order, as you can see here:
(This is no longer the case in Python 3.6, but note that it's not guaranteed behaviour yet.)
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> list(d)
['y', 'x', 'z']
>>> d.keys()
['y', 'x', 'z']
For your example, it is a better idea to use dict.items():
>>> d.items()
[('y', 2), ('x', 1), ('z', 3)]
This gives you a list of tuples. When you loop over them like this, each tuple is unpacked into k and v automatically:
for k,v in d.items():
print(k, 'corresponds to', v)
Using k and v as variable names when looping over a dict is quite common if the body of the loop is only a few lines. For more complicated loops it may be a good idea to use more descriptive names:
for letter, number in d.items():
print(letter, 'corresponds to', number)
It's a good idea to get into the habit of using format strings:
for letter, number in d.items():
print('{0} corresponds to {1}'.format(letter, number))

key is simply a variable.
For Python2.X:
>>> d = {'x': 1, 'y': 2, 'z': 3}
>>> for my_var in d:
>>> print my_var, 'corresponds to', d[my_var]
x corresponds to 1
y corresponds to 2
z corresponds to 3
... or better,
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.iteritems():
print the_key, 'corresponds to', the_value
For Python3.X:
d = {'x': 1, 'y': 2, 'z': 3}
for the_key, the_value in d.items():
print(the_key, 'corresponds to', the_value)

When you iterate through dictionaries using the for .. in ..-syntax, it always iterates over the keys (the values are accessible using dictionary[key]).
To iterate over key-value pairs, use the following:
for k,v in dict.iteritems() in Python 2
for k,v in dict.items() in Python 3

This is a very common looping idiom. in is an operator. For when to use for key in dict and when it must be for key in dict.keys() see David Goodger's Idiomatic Python article (archived copy).

I have a use case where I have to iterate through the dict to get the key, value pair, also the index indicating where I am. This is how I do it:
d = {'x': 1, 'y': 2, 'z': 3}
for i, (key, value) in enumerate(d.items()):
print(i, key, value)
Note that the parentheses around the key, value are important, without them, you'd get an ValueError "not enough values to unpack".

Iterating over dictionaries using 'for' loops
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
...
How does Python recognize that it needs only to read the key from the
dictionary? Is key a special word in Python? Or is it simply a
variable?
It's not just for loops. The important word here is "iterating".
A dictionary is a mapping of keys to values:
d = {'x': 1, 'y': 2, 'z': 3}
Any time we iterate over it, we iterate over the keys. The variable name key is only intended to be descriptive - and it is quite apt for the purpose.
This happens in a list comprehension:
>>> [k for k in d]
['x', 'y', 'z']
It happens when we pass the dictionary to list (or any other collection type object):
>>> list(d)
['x', 'y', 'z']
The way Python iterates is, in a context where it needs to, it calls the __iter__ method of the object (in this case the dictionary) which returns an iterator (in this case, a keyiterator object):
>>> d.__iter__()
<dict_keyiterator object at 0x7fb1747bee08>
We shouldn't use these special methods ourselves, instead, use the respective builtin function to call it, iter:
>>> key_iterator = iter(d)
>>> key_iterator
<dict_keyiterator object at 0x7fb172fa9188>
Iterators have a __next__ method - but we call it with the builtin function, next:
>>> next(key_iterator)
'x'
>>> next(key_iterator)
'y'
>>> next(key_iterator)
'z'
>>> next(key_iterator)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
When an iterator is exhausted, it raises StopIteration. This is how Python knows to exit a for loop, or a list comprehension, or a generator expression, or any other iterative context. Once an iterator raises StopIteration it will always raise it - if you want to iterate again, you need a new one.
>>> list(key_iterator)
[]
>>> new_key_iterator = iter(d)
>>> list(new_key_iterator)
['x', 'y', 'z']
Returning to dicts
We've seen dicts iterating in many contexts. What we've seen is that any time we iterate over a dict, we get the keys. Back to the original example:
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
If we change the variable name, we still get the keys. Let's try it:
>>> for each_key in d:
... print(each_key, '=>', d[each_key])
...
x => 1
y => 2
z => 3
If we want to iterate over the values, we need to use the .values method of dicts, or for both together, .items:
>>> list(d.values())
[1, 2, 3]
>>> list(d.items())
[('x', 1), ('y', 2), ('z', 3)]
In the example given, it would be more efficient to iterate over the items like this:
for a_key, corresponding_value in d.items():
print(a_key, corresponding_value)
But for academic purposes, the question's example is just fine.

For Iterating through dictionaries, The below code can be used.
dictionary= {1:"a", 2:"b", 3:"c"}
#To iterate over the keys
for key in dictionary.keys():
print(key)
#To Iterate over the values
for value in dictionary.values():
print(value)
#To Iterate both the keys and values
for key, value in dictionary.items():
print(key,'\t', value)

You can check the implementation of CPython's dicttype on GitHub. This is the signature of method that implements the dict iterator:
_PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey,
PyObject **pvalue, Py_hash_t *phash)
CPython dictobject.c

To iterate over keys, it is slower but better to use my_dict.keys(). If you tried to do something like this:
for key in my_dict:
my_dict[key+"-1"] = my_dict[key]-1
it would create a runtime error because you are changing the keys while the program is running. If you are absolutely set on reducing time, use the for key in my_dict way, but you have been warned.

If you are looking for a clear and visual example:
cat = {'name': 'Snowy', 'color': 'White' ,'age': 14}
for key , value in cat.items():
print(key, ': ', value)
Result:
name: Snowy
color: White
age: 14

This will print the output in sorted order by values in ascending order.
d = {'x': 3, 'y': 1, 'z': 2}
def by_value(item):
return item[1]
for key, value in sorted(d.items(), key=by_value):
print(key, '->', value)
Output:
y -> 1
z -> 2
x -> 3

Let's get straight to the point. If the word key is just a variable, as you have mentioned then the main thing to note is that when you run a 'FOR LOOP' over a dictionary it runs through only the 'keys' and ignores the 'values'.
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print (key, 'corresponds to', d[key])
rather try this:
d = {'x': 1, 'y': 2, 'z': 3}
for i in d:
print (i, 'corresponds to', d[i])
but if you use a function like:
d = {'x': 1, 'y': 2, 'z': 3}
print(d.keys())
in the above case 'keys' is just not a variable, its a function.

A dictionary in Python is a collection of key-value pairs. Each key is connected to a value, and you can use a key to access the value associated with that key. A key's value can be a number, a string, a list, or even another dictionary. In this case, threat each "key-value pair" as a separate row in the table: d is your table with two columns. the key is the first column, key[value] is your second column. Your for loop is a standard way to iterate over a table.

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