Useful Pandas Snippets

useful_pandas_snippets.py

#List unique values in a DataFrame column
pd.unique(df.column_name.ravel())

#Convert Series datatype to numeric, getting rid of any non-numeric values
df['col'] = df['col'].astype(str).convert_objects(convert_numeric=True)

#Grab DataFrame rows where column has certain values
valuelist = ['value1', 'value2', 'value3']
df = df[df.column.isin(value_list)]

#Grab DataFrame rows where column doesn't have certain values
valuelist = ['value1', 'value2', 'value3']
df = df[~df.column.isin(value_list)]

#Delete column from DataFrame
del df['column']

#Select from DataFrame using criteria from multiple columns
newdf = df[(df['column_one']>2004) & (df['column_two']==9)]

#Rename several DataFrame columns
df = df.rename(columns = {
    'col1 old name':'col1 new name',
    'col2 old name':'col2 new name',
    'col3 old name':'col3 new name',
})

#lower-case all DataFrame column names
df.columns = map(str.lower, df.columns)

#even more fancy DataFrame column re-naming
#lower-case all DataFrame column names (for example)
df.rename(columns=lambda x: x.split('.')[-1], inplace=True)

#Loop through rows in a DataFrame
#(if you must)
for index, row in df.iterrows():
    print index, row['some column']  

#Next few examples show how to work with text data in Pandas.
#Full list of .str functions: http://pandas.pydata.org/pandas-docs/stable/text.html

#Slice values in a DataFrame column (aka Series)
df.column.str[0:2]

#Lower-case everything in a DataFrame column
df.column_name = df.column_name.str.lower()

#Get length of data in a DataFrame column
df.column_name.str.len()

#Sort dataframe by multiple columns
df = df.sort(['col1','col2','col3'],ascending=[1,1,0])

#get top n for each group of columns in a sorted dataframe
#(make sure dataframe is sorted first)
top5 = df.groupby(['groupingcol1', 'groupingcol2']).head(5)

#Grab DataFrame rows where specific column is null/notnull
newdf = df[df['column'].isnull()]

#select from DataFrame using multiple keys of a hierarchical index
df.xs(('index level 1 value','index level 2 value'), level=('level 1','level 2'))

#Change all NaNs to None (useful before
#loading to a db)
df = df.where((pd.notnull(df)), None)

#Get quick count of rows in a DataFrame
len(df.index)

#Pivot data (with flexibility about what what
#becomes a column and what stays a row).
#Syntax works on Pandas >= .14
pd.pivot_table(
  df,values='cell_value',
  index=['col1', 'col2', 'col3'], #these stay as columns
  columns=['col4']) #data values in this column become their own column

#change data type of DataFrame column
df.column_name = df.column_name.astype(np.int64)

# Get rid of non-numeric values throughout a DataFrame:
for col in refunds.columns.values:
  refunds[col] = refunds[col].replace('[^0-9]+.-', '', regex=True)

#Set DataFrame column values based on other column values
df['column_to_change'][(df['column1'] == some_value) & (df['column2'] == some_other_value)] = new_value

#Clean up missing values in multiple DataFrame columns
df = df.fillna({
    'col1': 'missing',
    'col2': '99.999',
    'col3': '999',
    'col4': 'missing',
    'col5': 'missing',
    'col6': '99'
})

#Concatenate two DataFrame columns into a new, single column
#(useful when dealing with composite keys, for example)
df['newcol'] = df['col1'].map(str) + df['col2'].map(str)

#Doing calculations with DataFrame columns that have missing values
#In example below, swap in 0 for df['col1'] cells that contain null
df['new_col'] = np.where(pd.isnull(df['col1']),0,df['col1']) + df['col2']

# Split delimited values in a DataFrame column into two new columns
df['new_col1'], df['new_col2'] = zip(*df['original_col'].apply(lambda x: x.split(': ', 1)))

# Collapse hierarchical column indexes
df.columns = df.columns.get_level_values(0)

#Convert Django queryset to DataFrame
qs = DjangoModelName.objects.all()
q = qs.values()
df = pd.DataFrame.from_records(q)

#Create a DataFrame from a Python dictionary
df = pd.DataFrame(list(a_dictionary.items()), columns = ['column1', 'column2'])