scopes of local and global variable

[Pyenos]

#############################CODE##############################
t_len=0
class WORK:
    def getwork(self):
        def formattable(table_to_process,type):                                               
            TYPE=["p","t","T","s","i"] #list of types to format
            if type==TYPE[1]:
                def format_t():
                    row=[]
                    for col in table_to_process:

                        #######################
                        # ERROR PRONE PART    #
                        #######################
                        if len(str(col))>t_len:
                            t_len=len(str(col))
                        #######################
# Error message says:                                                   #
# UnboundLocalError: local variable 't_len' referenced before assignment#

                        row+=col
                        if (table_to_process.index(col)+1)%7==0:
                            t_temp.append(row)
                            row=[]
                format_t()
#################################################################

Interpreter says that t_len is local variable although i have
specified t_len=0 in line 1. Also, although i've stated t_len=0 in
line 1, it says that t_len is referenced before assignment.