Question

I’ve run into a strange problem while writing some code for my personal use. I’ll let my code do the talking…

def getValues(self, reader):
    for row in reader:
        #does stuff

    return assetName, efficiencyRating

def handleSave(self, assetName, reader):
    outputFile = open(self.outFilename, 'w')

    for row in reader:
        #does other stuff

    outputFile.close()
    return

def handleCalc(self):
    reader = csv.reader(open(self.filename), delimiter = ',', quotechar = '\"')

    assetName, efficiencyRating = self.getValues(reader)

    self.handleSave(assetName, reader)

This is just a portion of the code (obviously). The problem I’m having is in handleSave trying to loop through reader. It doesn’t appear to ever enter the loop? I’m really not sure what is happening. The loop in getValues behaves as expected.

Can someone explain what is happening? What have I done wrong? What should I do to fix this?

Answer 1

Once you’ve iterated through an iterator once, you can’t iterate through it again.

One way you can solve this is before you call handleSave, rewind the file and create a new reader:

f = open(self.filename)
reader = csv.reader(f, delimiter = ',', quotechar = '"')

assetName, efficiencyRating = self.getValues(reader)

f.seek(0) # rewind file
reader = csv.reader(f, delimiter = ',', quotechar = '"')

self.handleSave(assetName, reader)

Alternatively, you can read the data into a list:

rows = list(reader)

And then iterate through rows rather than reader.

As a side note, the convention in Python is for names to be lowercase, separated by underscores, rather than camel case. (e.g. get_values rather than getValues, handle_save rather than handleSave)