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--- python_dictionaries [2023/01/01 15:58] – [tasks] raju
+++ python_dictionaries [2023/08/02 19:41] (current) – [find key corresponding to a value] raju
@@ Line 1: / Line 1: @@
 ==== tasks ====
-group | related items, sort by | descending order of (nontriviality, interestingness, and usefulness)
+Policy: group | related items, sort by | descending order of (nontriviality, interestingness, and usefulness etc.,)
   * [[convert dict of dicts to list of dicts]]
   * [[Convert a dictionary of dataframes to a big dataframe]]
+  * [[Convert a string to a dictionary]]
+==== find key corresponding to a value ====
+The following will find the first key corresponding to a given value
+<code>
+>>> d ={1:'A', 2:'B', 3:'A', 4:'B'}
+>>> key_list = list(d.keys())
+>>> val_list = list(d.values())
+>>> lookup_val = 'A'
+>>> position = val_list.index(lookup_val)
+>>> key_val = key_list[position]
+>>> print(key_val)
+</code>
+<code>
+>>> lookup_val = 'B'
+>>> position = val_list.index(lookup_val)
+>>> key_val = key_list[position]
+>>> print(key_val)
+</code>
+Sample use case:
+  * In https://github.com/KamarajuKusumanchi/notebooks/blob/master/finvizfinance/target_price.ipynb , I used the above technique to find the index corresponding to 'Target Price'.
 ==== initialize a dictionary ====
@@ Line 14: / Line 39: @@
 Out[1]:
 {'sun': 1, 'mon': 2, 'tue': 3}
+</code>
+==== dump dictionary to file ====
+You can use pprint (stands for pretty print)
+<code>
+$ ipython
+Python 3.10.9 | packaged by conda-forge | (main, Jan 11 2023, 15:15:40) [MSC v.1916 64 bit (AMD64)]
+IPython 8.8.0 -- An enhanced Interactive Python. Type '?' for help.
+In [1]:
+import pprint
+cats = [{'name': 'Zophie', 'desc': 'chubby'}, {'name': 'Pooka', 'desc': 'fluffy'}]
+pprint.pformat(cats)
+Out[1]:
+"[{'desc': 'chubby', 'name': 'Zophie'}, {'desc': 'fluffy', 'name': 'Pooka'}]"
+In [2]:
+fileObj = open('myCats.py', 'w')
+fileObj.write('cats = ' + pprint.pformat(cats) + '\n')
+fileObj.close()
+In [3]:
+exit()
+</code>
+<code>
+$ cat myCats.py
+cats = [{'desc': 'chubby', 'name': 'Zophie'}, {'desc': 'fluffy', 'name': 'Pooka'}]
+</code>
+<code>
+$ ipython
+Python 3.10.9 | packaged by conda-forge | (main, Jan 11 2023, 15:15:40) [MSC v.1916 64 bit (AMD64)]
+IPython 8.8.0 -- An enhanced Interactive Python. Type '?' for help.
+In [1]:
+import myCats
+myCats.cats
+Out[1]:
+[{'desc': 'chubby', 'name': 'Zophie'}, {'desc': 'fluffy', 'name': 'Pooka'}]
+In [2]:
+myCats.cats[0]
+Out[2]:
+{'desc': 'chubby', 'name': 'Zophie'}
+In [3]:
+myCats.cats[0]['name']
+Out[3]:
+'Zophie'
+In [4]:
+exit()
+</code>
+Ref:-
+  * pprint is discussed in https://automatetheboringstuff.com/2e/chapter9/ -> Saving Variables with the pprint.pformat() Function
+==== merge python dictionaries ====
+To merge two python dictionaries
+<code>
+def merge_two_dicts(x, y):
+    '''
+    Given two dicts, merge them into a new dict as a shallow copy.
+    For common keys, the values in y take precedence over values in x.
+    '''
+    z = x.copy()
+    z.update(y)
+    return z
+</code>
+Sample usage:
+<code>
+>>> x = {'a': 1, 'b': 2}
+>>> y = {'b': 3, 'c': 4}
+>>> z = merge_two_dicts(x, y)
+>>> z
+{'a': 1, 'c': 4, 'b': 3}
+</code>
+To merge an undefined number of dicts
+<code>
+def merge_dicts(*dict_args):
+    '''
+    Given any number of dicts, shallow copy and merge into a new dict,
+    precedence goes to key value pairs in latter dicts.
+    '''
+    result = {}
+    for dictionary in dict_args:
+        result.update(dictionary)
+    return result
+</code>
+Given dicts a to g
+<code>
+z = merge_dicts(a, b, c, d, e, f, g)
+</code>
+will give a new dict z with all the key-value pairs. If same key exists in multiple dictionaries, the right most one will take precedence.
+Ref:- http://stackoverflow.com/questions/38987/how-to-merge-two-python-dictionaries-in-a-single-expression
+==== Convert a dictionary to a string ====
+<code>
+$ ipython
+Python 3.10.6 | packaged by conda-forge | (main, Oct 24 2022, 16:02:16) [MSC v.1916 64 bit (AMD64)]
+Type 'copyright', 'credits' or 'license' for more information
+IPython 8.4.0 -- An enhanced Interactive Python. Type '?' for help.
+In [1]:
+d = {'a':1, 'b':2}
+In [2]:
+import json
+s = json.dumps(d)
+In [3]:
+d
+Out[3]:
+{'a': 1, 'b': 2}
+In [4]:
+s
+Out[4]:
+'{"a": 1, "b": 2}'
+In [5]:
+type(d)
+Out[5]:
+dict
+In [6]:
+type(s)
+Out[6]:
+str
 </code>