python : convert the key of a dictionnary into a string : I would like to convert the keys of my dictionary as strings to print 
the title of my plot and to setup a path... 
I tried str(key) but it didn't worked... 

   
    dicohist = {    
    'Cost': df.costcheck1,
    'Cost2': df.costcheck2,}
    
    for key in dicohist:
        histo = Series.hist(dicohist[key],bins=300)	
        histo.set_xlabel("cost in dol")
        histo.set_ylabel("number of subject")
        histo.set_title(key)
        fig = histo.get_figure()
        fig.savefig('/path/%s.png'%(key))	