How to make grouper and axis the same length?

Question

For my assignment I'm supposed to plot the tracks of 20 hurricanes on a map using matplotlib. However when I run my code I get the error: AssertionError:Grouper and axis must be the same length

Here's the code I have:

import numpy as np
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt 
from PIL import *
fig = plt.figure(figsize=(12,12))

ax = fig.add_axes([0.1,0.1,0.8,0.8])

m = Basemap(llcrnrlon=-100.,llcrnrlat=0.,urcrnrlon=-20.,urcrnrlat=57.,
        projection='lcc',lat_1=20.,lat_2=40.,lon_0=-60.,
        resolution ='l',area_thresh=1000.)

m.bluemarble()
m.drawcoastlines(linewidth=0.5)
m.drawcountries(linewidth=0.5)
m.drawstates(linewidth=0.5)

# Creates parallels and meridians

m.drawparallels(np.arange(10.,35.,5.),labels=[1,0,0,1])
m.drawmeridians(np.arange(-120.,-80.,5.),labels=[1,0,0,1])
m.drawmapboundary(fill_color='aqua')

# Opens data file

import pandas as pd
name = [ ]
df = pd.read_csv('louisianastormb.csv')
for name, group in df.groupby([name]):
    latitude = group.lat.values
    longitude = group.lon.values
    x,y = m(longitude, latitude)
    plt.plot(x,y,'y-',linewidth=2 )
    plt.xlabel('Longitude')
    plt.ylabel('Latitude')
    plt.title('20 Hurricanes with Landfall in Louisiana')

plt.savefig('20hurpaths.jpg', dpi=100)

Here's the full error output:

 Traceback (most recent call last): 
File "/home/darealmzd/lstorms.py", line 31, in <module> 
for name, group in df.groupby([name]): 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/generic.py", line 186, in groupby 
squeeze=squeeze) 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.py", line 533, in groupby 
return klass(obj, by, **kwds) 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.py", line 197, in __init__ 
level=level, sort=sort) 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.py", line 1325, in _get_grouper 
ping = Grouping(group_axis, gpr, name=name, level=level, sort=sort) 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.py", line 1129, in __init__ 
self.grouper = _convert_grouper(index, grouper) 
File "/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.py", line 1350, in _convert_grouper 
raise Assertionerror('Grouper and axis must be same length') 
Assertionerror: Grouper and axis must be same length 

Answer 1

The problem is that you're grouping by (effectively) a list of empty list ([[]]). Because you have name = [] earlier and then you wrap that in a list as well.

If you want to group on a single column (called 'HurricaneName'), you should do something like:

for name, group in df.groupby('HurricaneName'):

However, if you want to group on multiple columns, then you need to pass a list:

for name, group in df.groupby(['HurricaneName', 'Year'])

If you want to put it in a variable like you have, you can do it like this:

col_name = 'State'

for name, group in df.groupby([col_name]):

Answer 2

ValueError: Grouper and axis must be same length

This can occur if you are using double brackets in the groupby argument.

(I posted this since it is the top result on Google).