how to declare query to fetch data from mongodb? - mongodb

Question

import pandas as pd
from pymongo import MongoClient
import matplotlib.pyplot as plt
def _connect_mongo(host, port, db):
""" A util for making a connection to mongo
if username and password:
mongo_uri = 'mongodb://%s:%s#%s:%s/%s' % (username, password, host, port, db)
conn = MongoClient(mongo_uri)
else:
"""
conn = MongoClient(host, port)
return conn[db]
def read_mongo(db, collection, host, port, query):
""" Read from Mongo and Store into DataFrame """
# Connect to MongoDB
db = _connect_mongo(host=host, port=port, db=db)
# Make a query to the specific DB and Collection
cursor = db[collection].find(query)
# Expand the cursor and construct the DataFrame
df = pd.DataFrame(list(cursor))
'''
Delete the _id
if no_id:
del df['_id']
'''
return df
#initialization
db = 'twittersmall'
collection='twitterdata'
query='{lang:{$exists: true}}'
host='localhost'
port=27017
var = read_mongo(db, collection, host, port, query)
print var
tweets_by_lang = var['lang'].value_counts()
fig, ax = plt.subplots()
ax.tick_params(axis='x', labelsize=15)
ax.tick_params(axis='y', labelsize=10)
ax.set_xlabel('Languages', fontsize=15)
ax.set_ylabel('Number of tweets' , fontsize=15)
ax.set_title('Top 5 languages', fontsize=15, fontweight='bold')
tweets_by_lang[:5].plot(ax=ax, kind='bar', color='red')
In this code, I was trying to fetch those data from mongodb where language field exists(might be null). So in the attribute, query I assigned a filter that will be used in the fetching operation. But the problem is, when I initialize query='{lang:{$exists: true}}', query is of string datatype and query must be a dictionary. When I declare query={lang:{$exists: true}} , it says -> syntax error. Obviously because so far as I know declaration of dictionary is {'key':'value'} . And when I declare like this query={'lang':'{$exists: true}'} it doesn't work because of keyError as there's filed in the database called lang.
So, how to declare this query and pass it in the method?
ps: when I use query={lang:{$exists: true}} in Webstorm terminal, it works, but I am currently working on jupyter notebook that integrates ipython, so that I can create graph, charts using the data from mongodb. I also used pandas for dataframe.