how to show a Reportlab report in the browser?

If what you want is to send the information flow of the PDF to the client for download, what you have to do is return a HttpResponse .

Example:

from django.http import HttpResponse

def exportarJornadasTrabajo(request):
    empresa = PlanearEmpresa.objects.all()[0]
    filename = "Reporte Jornadas de Trabajo %s.pdf" % empresa.razon_social
    # Creamos el response
    response = HttpResponse(content_type='application/pdf')
    response['Content-Disposition'] = 'attachment; filename="%s"' % filename
    # Observa que ahora en vez de usar el nombre del archivo usamos el response
    doc = SimpleDocTemplate(
        response,
        pagesize=letter, 
        rightMargin=72,
        leftMargin=72, 
        topMargin=72,
        bottomMargin=18
    )
    Story=[]
    # ...
    doc.build(Story) 
    # En vez de retornar un redirect retornamos el response
    return response

I have something similar to what you ask,

This is my HTML code in a Django Template and I do a POST to a URL that receives the parameter that I sent here

<div class="cuerpo"> 
    <div class="container">
        <div class="panel panel-default" style="width: auto;">
            <div class="panel-heading">Reporte Stock Productos por Almacen</div>
            <div class="panel-body">
                <form class="form form-horizontal" id='contra' name='contra' method='post' action='http://localhost:8000/reportestockalmacen/'>
                    <div class="row">
                        <div class="form-group">
                            <label for="cmbalmacen" class="col-sm-2 col-md-2 col-lg-2 control-label">Almacen</label>
                            <div class="col-sm-8 col-md-8 col-lg-6">
                                 <select class="form-control" name="cmbalmacen" id="cmbalmacen" required style="width:50%">
                                </select>
                            </div>
                            <div class="col-sm-2 col-md-2 col-lg-4">

                            </div>
                        </div>
                    </div>

                     <div class="row">
                        <div class="form-group">
                            <div class="col-sm-offset-2 col-sm-10">
                                <button type="reset" class="btn btn-default">Cancelar</button>                                
                                <input type="submit" id="button" name="button" class="btn btn-primary"  value="Aceptar"/>
                            </div>
                        </div>
                    </div>                        

                </form>

            </div>
        </div>
    </div>

  </div>

and this would be my Python code, which also has to be added to the URL.

def ReporteStockAlmacen(request):

if request.method == 'POST':        

    try:

        response = HttpResponse(content_type='ReporteProductosAlamcen/pdf')
        response['Content-Disposition'] = 'attachment; filename="Reporte_Productos_Almacen.pdf"'

        almacen=request.POST['cmbalmacen']                      
        jsonfinal = Get_ProductosPorAlmacen(almacen)

        data2 = jsonfinal["almacen"]

        temp = StringIO()
        width, height = A4
        p = canvas.Canvas(temp,pagesize = A4)   
        xPos = 50
        yPos = 685
        aum = 15
        pie = 60
        fecha = timezone.now().strftime('%Y-%m-%d %H:%M:%S')

        p.setFont("Helvetica",8)
        p.drawString(230, 730, "STOCK DE PRODUCTOS")
        p.drawString(215, 715, "Reporte generado el  "+str(fecha))
        p.line(700, 700, 0, 700)#(HORIZONTAL,,HORIZONTAL,)
        p.drawString(xPos, yPos, "CODIGO")
        p.drawString(xPos+120, yPos, "PRODUCTO")
        p.drawString(xPos+360, yPos, "SERIE")

        p.line(700, yPos-5, 0, yPos-5)                          

        for almacen in data2:

            p.drawString(500, 715, almacen['nombrealmacen'])

            if(yPos <= pie):
                p.showPage()
                p.save()
                xPos = 50
                yPos = 780
                aum = 15    
            x=0

            for productos in almacen["productos"]:
                p.setFont("Helvetica",8)

                if x == 0:
                    p.drawString(xPos, yPos-aum,productos['codigoproducto'])
                    p.drawString(xPos+120, yPos-aum,productos['nombreproducto'])
                    p.drawString(xPos+360, yPos-aum,productos['serieproducto_almacens'])
                    yPos-=aum


                    p.setFont("Helvetica",8)
                    if(yPos <= pie):

                        p.showPage()
                        p.save()

                        xPos = 50                       
                        yPos = 780
                        aum = 15                

        p.showPage()
        p.save()
        response.write(temp.getvalue())


        return response
    except:
        HttpResponse({'exito':0})